diff --git a/sys/include/sat.h b/sys/include/sat.h
new file mode 100644
index 000000000..ec3f86448
--- /dev/null
+++ b/sys/include/sat.h
@@ -0,0 +1,104 @@
+#pragma lib "libsat.a"
+
+typedef struct SATParam SATParam;
+typedef struct SATClause SATClause;
+typedef struct SATSolve SATSolve;
+typedef struct SATBlock SATBlock;
+typedef struct SATVar SATVar;
+typedef struct SATLit SATLit;
+typedef struct SATConflict SATConflict;
+
+/* user adjustable parameters */
+struct SATParam {
+	void (*errfun)(char *, void *);
+	void *erraux;
+	long (*randfn)(void *);
+	void *randaux;
+	
+	uint goofprob; /* probability of making a random decision, times 2**31 */
+	double varρ; /* Δactivity is multiplied by this after a conflict */
+	double clauseρ; /* Δclactivity is multiplied by this after a conflict */
+	int trivlim; /* number of extra literals we're willing to tolerate before substituting the trivial clause */
+	int purgeΔ; /* initial purge interval (number of conflicts before a purge) */
+	int purgeδ; /* increase in purge interval at purge */
+	double purgeα; /* α weight factor for purge heuristic */
+	u32int flushψ; /* agility threshold for restarts */
+};
+
+/* each block contains multiple SATClauses consecutively in its data region. each clause is 8 byte aligned and the total size is SATBLOCKSZ (64K) */
+struct SATBlock {
+	SATBlock *next, *prev;
+	SATClause *last; /* last clause, ==nil for empty blocks */
+	void *end; /* first byte past the last clause */
+	uchar data[1];
+};
+
+struct SATSolve {
+	SATParam;
+
+	uchar unsat; /* ==1 if unsatisfiable. don't even try to solve. */
+	uchar scratched; /* <0 if error happened, state undefined */
+
+	SATBlock bl[2]; /* two doubly linked list heads: list bl[0] contains user clauses, list bl[1] contains learned clauses */
+	SATBlock *lastbl; /* the last block we added a learned clause to */
+	SATClause *cl; /* all clauses are linked together; this is the first user clause */
+	SATClause *learncl; /* first learned clause */
+	SATClause **lastp[2]; /* this points towards the last link in each linked list */
+	int ncl; /* total number of clauses */
+	int ncl0; /* number of user clauses */
+	SATVar *var; /* all variables (array with nvar elements) */
+	SATLit *lit; /* all literals (array with 2*nvar elements) */
+	int nvar;
+	int nvaralloc; /* space allocated for variables */
+	int *trail; /* the trail contains all literals currently assumed true */
+	int ntrail;
+	int *decbd; /* decision boundaries. trail[decbd[i]] has the first literal of level i */
+	int lvl; /* current decision level */
+	SATVar **heap; /* binary heap with free variables, sorted by activity (nonfree variables are removed lazily and so may also be in it) */
+	int nheap;
+	
+	uint *lvlstamp; /* used to "stamp" levels during conflict resolution and purging */
+	uint stamp; /* current "stamp" counter */
+	
+	int forptr; /* trail[forptr] is the first literal we haven't explored the implications of yet */
+	int binptr; /* ditto for binary implications */
+	
+	int *cflcl; /* during conflict resolution we build the learned clause in here */
+	int ncflcl;
+	int cflclalloc; /* space allocated for cflcl */
+	int cfllvl; /* the maximum level of the literals in cflcl, cflcl[0] excluded */
+	int cflctr; /* number of unresolved literals during conflict resolution */
+	
+	double Δactivity; /* activity increment for variables */
+	double Δclactivity; /* activity increment for clauses */
+	
+	uvlong conflicts; /* total number of conflicts so far */
+	
+	uvlong nextpurge; /* purge happens when conflicts >= nextpurge */
+	uint purgeival; /* increment for nextpurge */
+	/* during a purge we do a "full run", assigning all variables and recording conflicts rather than resolving them */
+	SATConflict *fullrcfl; /* conflicts found thus */
+	int nfullrcfl;
+	int fullrlvl; /* minimum cfllvl for conflicts found during purging */
+	int *fullrlits; /* literals implied by conflicts at level fullrlvl */
+	int nfullrlits;
+	int rangecnt[256]; /* number of clauses with certain range values */
+	
+	u64int nextflush; /* flush happens when conflicts >= nextflush */
+	u32int flushu, flushv, flushθ; /* variables for flush scheduling algorithm */
+	u32int agility; /* agility is a measure how quickly variables are being flipped. high agility inhibits flushes */
+	
+	void *scrap; /* auxiliary memory that may need to be freed after a fatal error */
+};
+
+SATSolve *satnew(void);
+SATSolve *satadd1(SATSolve *, int *, int);
+SATSolve *sataddv(SATSolve *, ...);
+SATSolve *satrange1(SATSolve *, int *, int, int, int);
+SATSolve *satrangev(SATSolve *, int, int, ...);
+int satsolve(SATSolve *);
+int satmore(SATSolve *);
+int satval(SATSolve *, int);
+void satfree(SATSolve *);
+void satreset(SATSolve *);
+
diff --git a/sys/man/2/sat b/sys/man/2/sat
new file mode 100644
index 000000000..7a14b6ae6
--- /dev/null
+++ b/sys/man/2/sat
@@ -0,0 +1,231 @@
+.TH SAT 2
+.SH NAME
+satnew, satadd1, sataddv, satrange1, satrangev, satsolve, satmore, satval, satreset, satfree \- boolean satisfiability (SAT) solver
+.SH SYNOPSIS
+.de PB
+.PP
+.ft L
+.nf
+..
+.PB
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+.PB
+struct SATParam {
+	void (*errfun)(char *msg, void *erraux);
+	void *erraux;
+	long (*randfn)(void *randaux);
+	void *randaux;
+	/* + finetuning parameters, see sat.h */
+};
+.PB
+struct SATSolve {
+	SATParam;
+	/* + internals */
+};
+.PB
+.ta +\w'\fLSATSolve* \fP'u
+SATSolve*	satnew(void);
+void	satfree(SATSolve *s);
+SATSolve*	satadd1(SATSolve *s, int *lit, int nlit);
+SATSolve*	sataddv(SATSolve *s, ...);
+SATSolve*	satrange1(SATSolve *s, int *lit, int nlit,
+			int min, int max);
+SATSolve*	satrangev(SATSolve *s, int min, int max, ...);
+int	satsolve(SATSolve *s);
+int	satmore(SATSolve *s);
+int	satval(SATSolve *s, int lit);
+void	satreset(SATSolve *s);
+.SH DESCRIPTION
+.PP
+.I Libsat
+is a solver for the boolean satisfiability problem, i.e. given a boolean formula it will either find an assignment to the variables that makes it true, or report that this is impossible.
+The input formula must be in conjunctive normal form (CNF), i.e. of the form
+.IP
+.if t (x\d\s71\s10\u ∨ x\d\s72\s10\u ∨ x\d\s73\s10\u ∨ …) ∧ (y\d\s71\s10\u ∨ y\d\s72\s10\u ∨ y \d\s73\s10\u ∨ …) ∧ …,
+.if n (x1 ∨ x2 ∨ x3 ∨ ...) ∧ (y1 ∨ y2 ∨ y3 ∨ ...) ∧ ...,
+.PP
+where each
+.if t x\d\s7i\s10\u or y\d\s7i\s10\u
+.if n x_i or y_i
+can optionally be negated.
+.PP
+For example, consider
+.IP
+.if t (x\d\s71\s10\u ∨ x\d\s72\s10\u ∨ x\d\s73\s10\u) ∧ (¬x\d\s71\s10\u ∨ ¬x\d\s72\s10\u) ∧ (¬x\d\s72\s10\u ∨ ¬x\d\s73\s10\u) ∧ (¬x\d\s71\s10\u ∨ ¬x\d\s73\s10\u).
+.if n (x1 ∨ x2 ∨ x3) ∧ (¬x1 ∨ ¬x2) ∧ (¬x2 ∨ ¬x3) ∧ (¬x1 ∨ ¬x3).
+.PP
+This formula encodes the constraint that exactly one of the three variables be true. To represent this as input for
+.I libsat
+we assign positive integers to each variable.
+Negation is represented by the corresponding negative number, hence our example corresponds to the set of "clauses"
+.IP
+1, 2, 3
+.br
+-1, -2
+.br
+-1, -3
+.br
+-2, -3
+.PP
+To actually solve this problem we would create a
+.B SATSolve
+structure and add clauses one by one using
+.I satadd1
+or
+.I sataddv
+(the former takes an
+.B int
+array, the latter a variadic list terminated by 0).
+The
+.B SATSolve
+is modified inplace but returned for convenience.
+Passing
+.B nil
+as a first argument will create and return a new structure.
+Alternatively,
+.I satnew
+will create an empty structure.
+.PP
+Once clauses have been added,
+.I satsolve
+will invoke the actual solver.
+It returns 1 if it found an assignment and 0 if there is no assignment (the formula is unsatisfiable).
+If an assignment has been found, further clauses can be added to constrain it further and
+.I satsolve
+rerun.
+.I Satmore
+performs this automatically, excluding the current values of the variables.
+It is equivalent to
+.I satsolve
+if no variables have assigned values.
+.PP
+Once a solution has been found,
+.I satval
+returns the value of literal
+.I lit.
+It returns 1 for true, 0 for false and -1 for undetermined.
+If the formula is satisfiable, an undetermined variable is one where either value will satisfy the formula.
+If the formula is unsatisfiable, all variables are undetermined.
+.PP
+.I Satrange1
+and
+.I satrangev
+function like their
+.I satadd
+brethren but rather than adding a single clause they add multiple clauses corresponding to the constraint that at least
+.I min
+and at most
+.I max
+literals from the provided array be true.
+For example, the clause from above corresponds to
+.IP
+.B "satrangev(s, 1, 1, 1, 2, 3, 0);"
+.PP
+.I Satreset
+resets all solver state, deleting all learned clauses and variable assignments.
+It retains all user provided clauses.
+.I Satfree
+deletes a solver structure and frees all associated storage.
+.PP
+There are a number of user-adjustable parameters in the 
+.B SATParam
+structure embedded in
+.BR SATSolve .
+.I Randfun
+is called with argument
+.I randaux
+to generate random numbers between 0 and
+.if t 2\u\s731\s10\d-1;
+.if n 2^31-1;
+it defaults to
+.IR lrand (2).
+.I Errfun
+is called on fatal errors (see DIAGNOSTICS).
+Additionally, a number of finetuning parameters are defined in
+.BR sat.h .
+By tweaking their values, the run-time for a given problem can be reduced.
+.SH EXAMPLE
+Find all solutions to the example clause from above:
+.PB
+.ta .5i 1i 1.5i
+	SATSolve *s;
+	
+	s = nil;
+	s = sataddv(s, 1, 2, 3, 0);
+	s = sataddv(s, -1, -2, 0);
+	s = sataddv(s, -1, -3, 0);
+	s = sataddv(s, -2, -3, 0);
+	while(satmore(s) > 0)
+		print("x1=%d x2=%d x3=%d\\n",
+			satval(s, 1),
+			satval(s, 2),
+			satval(s, 3));
+	satfree(s);
+.SH SOURCE
+.B /sys/src/libsat
+.SH "SEE ALSO"
+Donald Knuth, ``The Art of Computer Programming'', Volume 4, Fascicle 6.
+.SH DIAGNOSTICS
+.I Satnew
+returns
+.B nil
+on certain fatal error conditions (such as
+.IR malloc (2)
+failure).
+Other routines will call
+.I errfun
+with an error string and
+.IR erraux .
+If no
+.I errfun
+is provided or if it returns,
+.IR sysfatal (2)
+is called.
+It is permissible to use
+.IR setjmp (2)
+to return from an error condition.
+Call
+.I satfree
+to clean up the
+.B SATSolve
+structure in this case.
+Note that calling the
+.I satadd
+or
+.I satrange
+routines with
+.B nil
+first argument will invoke
+.I sysfatal
+on error, since no
+.I errfun
+has been defined yet.
+.SH BUGS
+Variable numbers should be consecutive numbers starting from 1, since variable data is kept in arrays internally.
+.PP
+Large clauses of several thousand literals are probably inefficient and should be split up using auxiliary variables.
+Very large clauses exceeding about 16,000 literals will not work at all.
+.PP
+There is no way to remove clauses (since it's unclear what the semantics should be).
+.PP
+The details about the tuning parameters are subject to change.
+.PP
+Calling
+.I satadd
+or
+.I satrange
+after 
+.I satsolve
+or
+.I satmore
+may reset variable values.
+.PP
+.I Satmore
+will always return 1 when there are no assigned variables in the solution.
+.P
+Some debugging routines called under "shouldn't happen" conditions are non-reentrant.
+.SH HISTORY
+.I Libsat
+first appeared in 9front in March, 2018.
diff --git a/sys/src/libsat/debug.c b/sys/src/libsat/debug.c
new file mode 100644
index 000000000..0f9ffe30f
--- /dev/null
+++ b/sys/src/libsat/debug.c
@@ -0,0 +1,130 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+static SATSolve *globsat;
+
+static int
+satclausefmt(Fmt *f)
+{
+	SATClause *c;
+	char *s;
+	int i, fl;
+	
+	fl = f->flags;
+	c = va_arg(f->args, SATClause *);
+	if(c == nil)
+		return fmtstrcpy(f, "Λ");
+	if(c->n == 0)
+		return fmtstrcpy(f, "ε");
+	s = "%s%d";
+	for(i = 0; i < c->n; i++){
+		if((fl & FmtSign) != 0)
+			switch(globsat->lit[c->l[i]].val){
+			case 1: s = "%s[%d]"; break;
+			case 0: s = "%s(%d)"; break;
+			case -1: s = "%s%d"; break;
+			default: abort();
+			}
+		fmtprint(f, s, i != 0 ? " ∨ " : "", signf(c->l[i]));
+	}
+	return 0;
+}
+
+void
+satprintstate(SATSolve *s)
+{
+	int i;
+	Fmt f;
+	char buf[512];
+	SATVar *v;
+	
+	fmtfdinit(&f, 1, buf, sizeof(buf));
+	fmtprint(&f, "trail:\n");
+	for(i = 0; i < s->ntrail; i++){
+		v = &s->var[VAR(s->trail[i])];
+		fmtprint(&f, "%c%-8d %- 8d %-8d ", i == s->forptr ? '*' : ' ', i, signf(s->trail[i]), v->lvl);
+		if(v->isbinreason)
+			fmtprint(&f, "%d ∨ %d\n", signf(s->trail[i]), signf(v->binreason));
+		else
+			fmtprint(&f, "%+Γ\n", v->reason);
+	}
+	fmtrune(&f, '\n');
+	fmtfdflush(&f);
+}
+
+void
+satsanity(SATSolve *s)
+{
+	int i, j, k, m, tl, s0, s1;
+	SATVar *v;
+	SATLit *l;
+	SATClause *c;
+	
+	for(c = s->cl; c != nil; c = c->next){
+		assert(c->n >= 2);
+		assert((uint)((uchar*)c->next - (uchar*)c) >= sizeof(SATClause) + (c->n - 1) * sizeof(int));
+		for(j = 0; j < c->n; j++)
+			assert((uint)c->l[j] < 2*s->nvar);
+		for(i = 0; i < 2; i++)
+			c->watch[i] = (void*)((uintptr)c->watch[i] | 1);
+	}
+	for(i = 0; i < s->nvar; i++){
+		tl = -1;
+		for(j = 0; j < s->ntrail; j++)
+			if(VAR(s->trail[j]) == i){
+				assert(tl == -1);
+				tl = j;
+			}
+		v = &s->var[i];
+		l = &s->lit[2*i];
+		if(l->val >= 0){
+			assert(l->val <= 1);
+			assert(l[0].val + l[1].val == 1);
+			assert((uint)v->lvl <= s->lvl);
+			assert(tl != -1);
+			assert(s->trail[tl] == 2*i+l[1].val);
+			assert(tl >= s->decbd[v->lvl]);
+			assert(v->lvl == s->lvl || tl < s->decbd[v->lvl+1]);
+		}else{
+			assert(l[0].val == -1 && l[1].val == -1);
+			assert(v->lvl == -1);
+			assert(v->heaploc >= 0);
+			assert(tl == -1);
+		}
+		assert(v->heaploc == -1 || (uint)v->heaploc <= s->nheap && s->heap[v->heaploc] == v);
+		for(j = 0; j < 2; j++){
+			m = 2 * i + j;
+			for(c = l[j].watch; c != nil; c = c->watch[k]){
+				k = c->l[1] == m;
+				assert(k || c->l[0] == m);
+				assert((uintptr)c->watch[k] & 1);
+				c->watch[k] = (void*)((uintptr)c->watch[k] & ~1);
+			}
+		}
+	}
+	for(c = s->cl; c != nil; c = c->next)
+		for(i = 0; i < 2; i++)
+			assert(((uintptr)c->watch[i] & 1) == 0);
+	if(s->forptr == s->ntrail)
+		for(c = s->cl; c != nil; c = c->next){
+			s0 = s->lit[c->l[0]].val;
+			s1 = s->lit[c->l[1]].val;
+			if(s0 != 0 && s1 != 0 || s0 == 1 || s1 == 1)
+				continue;
+			for(i = 2; i < c->n; i++)
+				if(s->lit[c->l[i]].val != 0){
+					satprintstate(s);
+					print("watchlist error: %+Γ\n", c);
+					assert(0);
+				}
+		}
+}
+
+void
+satdebuginit(SATSolve *s)
+{
+	globsat = s;
+	fmtinstall(L'Γ', satclausefmt);
+}
diff --git a/sys/src/libsat/impl.h b/sys/src/libsat/impl.h
new file mode 100644
index 000000000..9049299b7
--- /dev/null
+++ b/sys/src/libsat/impl.h
@@ -0,0 +1,81 @@
+/* note that internally, literals use a representation different from the API.
+ * variables are numbered from 0 (not 1) and 2v and 2v+1 correspond to v
+ * and ¬v, resp.  */
+#define VAR(l) ((l)>>1)
+#define NOT(l) ((l)^1)
+
+/* l[0] and l[1] are special: they are the watched literals.
+ * all clauses that have literal l on their watchlist form a linked list starting with s->lit[l].watch
+ * and watch[i] having the next clause for l[i] */
+struct SATClause {
+	SATClause *next;
+	SATClause *watch[2];
+	double activity; /* activity is increased every time a clause is used to resolve a conflict (tiebreaking heuristic during purging) */
+	int n; /* >= 2 for learned clauses and > 2 for input clauses (binary input clauses are kept in the bimp tables) */
+	ushort range; /* heuristic used during purging, low range => keep clause (range 0..256) */
+	int l[1];
+};
+
+struct SATLit {
+	int *bimp; /* array of literals implied by this literal through binary clauses (Binary IMPlications) */
+	SATClause *watch; /* linked list of watched clauses */
+	int nbimp;
+	char val; /* -1 = not assigned, 0 = false, 1 = true */
+};
+
+struct SATVar {
+	double activity; /* activity is increased every time a variable shows up in a conflict */
+	union {
+		SATClause *reason; /* nil for decision and free literals */
+		int binreason; /* used when isbinreason == 1: the reason is the clause l ∨ l->binreason */
+	};
+	int lvl; /* level at which this variable is defined, or -1 for free variables */
+	int heaploc; /* location in binary heap or -1 when not in heap */
+	uint stamp; /* "stamp" value used for conflict resolution etc. */
+	uchar flags; /* see below */
+	char isbinreason;
+};
+
+enum {
+	VARPHASE = 1, /* for a free variables, VARPHASE is used as a first guess the next time it is picked */
+	VARUSER = 0x80, /* user assigned variable (unit clause in input) */
+};
+
+/* records conflicts during purging */
+struct SATConflict {
+	union {
+		SATClause *c;
+		uvlong b;
+	};
+	int lvl; /* bit 31 denotes binary conflict */
+};
+#define CFLLVL(c) ((c).lvl & 0x7fffffff)
+
+enum {
+	SATBLOCKSZ = 65536,
+	SATVARALLOC = 64,
+	CLAUSEALIGN = 8,
+	CFLCLALLOC = 16,
+};
+
+void saterror(SATSolve *, char *, ...);
+void sataddtrail(SATSolve *, int);
+void satdebuginit(SATSolve *);
+void satprintstate(SATSolve *);
+void satsanity(SATSolve *);
+SATVar *satheaptake(SATSolve *);
+void satheapput(SATSolve *, SATVar *);
+void satreheap(SATSolve *, SATVar *);
+void satheapreset(SATSolve *);
+int satnrand(SATSolve *, int);
+void *satrealloc(SATSolve *, void *, ulong);
+SATClause *satnewclause(SATSolve *, int, int);
+SATClause *satreplclause(SATSolve *, int);
+void satcleanup(SATSolve *, int);
+void satbackjump(SATSolve *, int);
+
+#define signf(l) (((l)<<31>>31|1)*((l)+2>>1))
+#pragma varargck type "Γ" SATClause*
+
+#define ε 2.2250738585072014e-308
+#define MAXACTIVITY 1e100
diff --git a/sys/src/libsat/misc.c b/sys/src/libsat/misc.c
new file mode 100644
index 000000000..e7e843529
--- /dev/null
+++ b/sys/src/libsat/misc.c
@@ -0,0 +1,206 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+SATSolve *
+satnew(void)
+{
+	SATSolve *s;
+	
+	s = calloc(1, sizeof(SATSolve));
+	if(s == nil) return nil;
+	s->bl[0].next = s->bl[0].prev = &s->bl[0];
+	s->bl[1].next = s->bl[1].prev = &s->bl[1];
+	s->bl[0].end = (uchar*)&s->bl[0] + SATBLOCKSZ; /* this block is "full" */
+	s->bl[1].end = (uchar*)&s->bl[1] + SATBLOCKSZ;
+	s->lastp[0] = &s->cl;
+	s->lastp[1] = &s->learncl;
+	s->lastbl = &s->bl[1];
+	s->randfn = (long(*)(void*)) lrand;
+	
+	s->goofprob = 0.02 * (1UL<<31);
+	s->varρ = 1/0.9;
+	s->clauseρ = 1/0.999;
+	s->trivlim = 10;
+	s->purgeΔ = 10000;
+	s->purgeδ = 100;
+	s->purgeα = 0.2;
+	s->flushψ = (1ULL<<32)*0.05;
+	
+	s->Δactivity = 1;
+	s->Δclactivity = 1;
+	
+	return s;
+}
+
+void
+satfree(SATSolve *s)
+{
+	SATBlock *b, *bb;
+	int i;
+	
+	if(s == nil) return;
+	for(i = 0; i < 2; i++)
+		for(b = s->bl[i].next; b != &s->bl[i]; b = bb){
+			bb = b->next;
+			free(b);
+		}
+	for(i = 0; i < 2 * s->nvar; i++)
+		free(s->lit[i].bimp);
+	free(s->heap);
+	free(s->trail);
+	free(s->decbd);
+	free(s->var);
+	free(s->lit);
+	free(s->cflcl);
+	free(s->fullrcfl);
+	free(s->fullrlits);
+	free(s->scrap);
+	free(s);
+}
+
+void
+saterror(SATSolve *s, char *msg, ...)
+{
+	char buf[ERRMAX];
+	va_list va;
+	
+	va_start(va, msg);
+	vsnprint(buf, sizeof(buf), msg, va);
+	va_end(va);
+	s->scratched = 1;
+	if(s != nil && s->errfun != nil)
+		s->errfun(buf, s->erraux);
+	sysfatal("%s", buf);
+}
+
+int
+satval(SATSolve *s, int l)
+{
+	int m, v;
+	
+	if(s->unsat) return -1;
+	m = l >> 31;
+	v = (l + m ^ m) - 1;
+	if(v < 0 || v >= s->nvar) return -1;
+	return s->lit[2*v+(m&1)].val;
+}
+
+int
+satnrand(SATSolve *s, int n)
+{
+	long slop, v;
+
+	if(n <= 1) return 0;
+	slop = 0x7fffffff % n;
+	do
+		v = s->randfn(s->randaux);
+	while(v <= slop);
+	return v % n;
+}
+
+void *
+satrealloc(SATSolve *s, void *v, ulong n)
+{
+	v = realloc(v, n);
+	if(v == nil)
+		saterror(s, "realloc: %r");
+	return v;
+}
+
+#define LEFT(x) (2*(x)+1)
+#define RIGHT(x) (2*(x)+2)
+#define UP(x) ((x)-1>>1)
+
+static SATVar *
+heapswap(SATSolve *s, int i, int j)
+{
+	SATVar *r;
+	
+	if(i == j) return s->heap[i];
+	r = s->heap[i];
+	s->heap[i] = s->heap[j];
+	s->heap[j] = r;
+	s->heap[i]->heaploc = i;
+	s->heap[j]->heaploc = j;
+	return r;
+}
+
+static void
+heapup(SATSolve *s, int i)
+{
+	int m;
+	
+	m = i;
+	for(;;){
+		if(LEFT(i) < s->nheap && s->heap[LEFT(i)]->activity > s->heap[m]->activity)
+			m = LEFT(i);
+		if(RIGHT(i) < s->nheap && s->heap[RIGHT(i)]->activity > s->heap[m]->activity)
+			m = RIGHT(i);
+		if(i == m) break;
+		heapswap(s, m, i);
+		i = m;
+	}
+}
+
+static void
+heapdown(SATSolve *s, int i)
+{
+	int p;
+
+	for(; i > 0 && s->heap[p = UP(i)]->activity < s->heap[i]->activity; i = p)
+		heapswap(s, i, p);
+}
+
+SATVar *
+satheaptake(SATSolve *s)
+{
+	SATVar *r;
+	
+	assert(s->nheap > 0);
+	r = heapswap(s, 0, --s->nheap);
+	heapup(s, 0);
+	r->heaploc = -1;
+	return r;
+}
+
+void
+satheapput(SATSolve *s, SATVar *v)
+{
+	assert(s->nheap < s->nvar);
+	v->heaploc = s->nheap;
+	s->heap[s->nheap++] = v;
+	heapdown(s, s->nheap - 1);
+}
+
+void
+satreheap(SATSolve *s, SATVar *v)
+{
+	int i;
+	
+	i = v->heaploc;
+	if(i < 0) return;
+	heapup(s, i);
+	heapdown(s, i);
+}
+
+void
+satheapreset(SATSolve *s)
+{
+	int i, n, j;
+	
+	s->heap = satrealloc(s, s->heap, s->nvar * sizeof(SATVar *));
+	n = s->nvar;
+	s->nheap = n;
+	for(i = 0; i < n; i++){
+		s->heap[i] = &s->var[i];
+		s->heap[i]->heaploc = i;
+	}
+	for(i = 0; i < n - 1; i++){
+		j = i + satnrand(s, n - i);
+		heapswap(s, i, j);
+		heapdown(s, i);
+	}
+	heapdown(s, n - 1);
+}
diff --git a/sys/src/libsat/mkfile b/sys/src/libsat/mkfile
new file mode 100644
index 000000000..08b1639e4
--- /dev/null
+++ b/sys/src/libsat/mkfile
@@ -0,0 +1,22 @@
+</$objtype/mkfile
+
+LIB=/$objtype/lib/libsat.a
+
+OFILES=\
+	misc.$O \
+	satadd.$O \
+	satrange.$O \
+	satsolve.$O \
+	satmore.$O \
+	debug.$O \
+
+HFILES=\
+	/sys/include/sat.h\
+	impl.h\
+
+UPDATE=\
+	mkfile\
+	$HFILES\
+	${OFILES:%.$O=%.c}\
+
+</sys/src/cmd/mksyslib
diff --git a/sys/src/libsat/satadd.c b/sys/src/libsat/satadd.c
new file mode 100644
index 000000000..3a0cf0c3a
--- /dev/null
+++ b/sys/src/libsat/satadd.c
@@ -0,0 +1,230 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+static SATBlock *
+newblock(SATSolve *s, int learned)
+{
+	SATBlock *b;
+	
+	b = calloc(1, SATBLOCKSZ);
+	if(b == nil)
+		saterror(s, "malloc: %r");
+	b->prev = s->bl[learned].prev;
+	b->next = &s->bl[learned];
+	b->next->prev = b;
+	b->prev->next = b;
+	b->end = (void*) b->data;
+	return b;
+}
+
+SATClause *
+satnewclause(SATSolve *s, int n, int learned)
+{
+	SATBlock *b;
+	SATClause *c;
+	int f, sz;
+	
+	sz = sizeof(SATClause) + (n - 1) * sizeof(int);
+	assert(sz <= SATBLOCKSZ);
+	if(learned)
+		b = s->lastbl;
+	else
+		b = s->bl[0].prev;
+	for(;;){
+		f = (uchar*)b + SATBLOCKSZ - (uchar*)b->end;
+		if(f >= sz) break;
+		b = b->next;
+		if(b == &s->bl[learned])
+			b = newblock(s, learned);
+	}
+	c = b->end;
+	memset(c, 0, sizeof(SATClause));
+	b->end = (void *)((uintptr)b->end + sz + CLAUSEALIGN - 1 & -CLAUSEALIGN);
+	b->last = c;
+	if(learned){
+		if(s->lastp[1] == &s->learncl)
+			*s->lastp[0] = c;
+		s->lastbl = b;
+	}else
+		c->next = s->learncl;
+	*s->lastp[learned] = c;
+	s->lastp[learned] = &c->next;
+	s->ncl++;
+	return c;
+}
+
+/* this is currently only used to subsume clauses, i.e. n is guaranteed to be less than the last n */
+SATClause *
+satreplclause(SATSolve *s, int n)
+{
+	SATBlock *b;
+	SATClause *c, **wp;
+	int f, sz, i, l;
+	
+	assert(s->lastbl != nil && s->lastbl->last != nil);
+	b = s->lastbl;
+	c = b->last;
+	f = (uchar*)b + SATBLOCKSZ - (uchar*)c;
+	sz = sizeof(SATClause) + (n - 1) * sizeof(int);
+	assert(f >= sz);
+	b->end = (void *)((uintptr)c + sz + CLAUSEALIGN - 1 & -CLAUSEALIGN);
+	for(i = 0; i < 2; i++){
+		l = c->l[i];
+		for(wp = &s->lit[l].watch; *wp != nil && *wp != c; wp = &(*wp)->watch[(*wp)->l[1] == l])
+			;
+		assert(*wp != nil);
+		*wp = c->watch[i];
+	}
+	memset(c, 0, sizeof(SATClause));
+	return c;
+}
+
+static int
+litconv(SATSolve *s, int l)
+{
+	int v, m, n;
+	SATVar *vp;
+	SATLit *lp;
+	
+	m = l >> 31;
+	v = (l + m ^ m) - 1;
+	if(v >= s->nvaralloc){
+		n = -(-(v+1) & -SATVARALLOC);
+		s->var = vp = satrealloc(s, s->var, n * sizeof(SATVar));
+		s->lit = lp = satrealloc(s, s->lit, 2 * n * sizeof(SATLit));
+		memset(vp += s->nvaralloc, 0, (n - s->nvaralloc) * sizeof(SATVar));
+		memset(lp += 2*s->nvaralloc, 0, 2 * (n - s->nvaralloc) * sizeof(SATLit));
+		for(; vp < s->var + n; vp++){
+			vp->lvl = -1;
+			vp->flags = VARPHASE;
+		}
+		for(; lp < s->lit + 2 * n; lp++)
+			lp->val = -1;
+		s->nvaralloc = n;
+	}
+	if(v >= s->nvar)
+		s->nvar = v + 1;
+	return v << 1 | m & 1;
+}
+
+static void
+addbimp(SATSolve *s, int l0, int l1)
+{
+	SATLit *lp;
+	
+	lp = &s->lit[NOT(l0)];
+	lp->bimp = satrealloc(s, lp->bimp, (lp->nbimp + 1) * sizeof(int));
+	lp->bimp[lp->nbimp++] = l1;
+}
+
+static SATSolve *
+satadd1special(SATSolve *s, int *a, int n)
+{
+	int i, l0, l1;
+	
+	if(n == 0){
+		s->unsat = 1;
+		return s;
+	}
+	l0 = a[0];
+	l1 = 0;
+	for(i = 1; i < n; i++)
+		if(a[i] != l0){
+			l1 = a[i];
+			break;
+		}
+	if(l1 == 0){
+		l0 = litconv(s, l0);
+		assert(s->lvl == 0);
+		switch(s->lit[l0].val){
+		case 0:
+			s->unsat = 1;
+			return s;
+		case -1:
+			s->trail = satrealloc(s, s->trail, sizeof(int) * s->nvar);
+			memmove(&s->trail[1], s->trail, sizeof(int) * s->ntrail);
+			s->trail[0] = l0;
+			s->ntrail++;
+			s->var[VAR(l0)].flags |= VARUSER;
+			s->var[VAR(l0)].lvl = 0;
+			s->lit[l0].val = 1;
+			s->lit[NOT(l0)].val = 0;
+		}
+		return s;
+	}
+	if(l0 + l1 == 0) return s;
+	l0 = litconv(s, l0);
+	l1 = litconv(s, l1);
+	addbimp(s, l0, l1);
+	addbimp(s, l1, l0);
+	return s;
+}
+
+SATSolve *
+satadd1(SATSolve *s, int *a, int n)
+{
+	SATClause *c;
+	int i, j, l, u;
+	SATVar *v;
+
+	if(s == nil){
+		s = satnew();
+		if(s == nil)
+			saterror(nil, "satnew: %r");
+	}
+	if(n < 0)
+		for(n = 0; a[n] != 0; n++)
+			;
+	for(i = 0; i < n; i++)
+		if(a[i] == 0)
+			saterror(s, "satadd1(%p, %p, %d): a[%d]==0, callerpc=%p", s, a, n, i, getcallerpc(&s));
+	satbackjump(s, 0);
+	if(n <= 2)
+		return satadd1special(s, a, n);
+	/* use stamps to detect repeated literals and tautological clauses */
+	if(s->stamp >= (uint)-6){
+		for(i = 0; i < s->nvar; i++)
+			s->var[i].stamp = 0;
+		s->stamp = 1;
+	}else
+		s->stamp += 3;
+	u = 0;
+	for(i = 0; i < n; i++){
+		l = litconv(s, a[i]);
+		v = &s->var[VAR(l)];
+		if(v->stamp < s->stamp) u++;
+		if(v->stamp == s->stamp + (~l & 1))
+			return s; /* tautological */
+		v->stamp = s->stamp + (l & 1);
+	}
+	if(u <= 2)
+		return satadd1special(s, a, n);
+	s->stamp += 3;
+	c = satnewclause(s, u, 0);
+	c->n = u;
+	for(i = 0, j = 0; i < n; i++){
+		l = litconv(s, a[i]);
+		v = &s->var[VAR(l)];
+		if(v->stamp < s->stamp){
+			c->l[j++] = l;
+			v->stamp = s->stamp;
+		}
+	}
+	assert(j == u);
+	s->ncl0++;
+	return s;
+}
+
+SATSolve *
+sataddv(SATSolve *s, ...)
+{
+	va_list va;
+	
+	va_start(va, s);
+	/* horrible hack */
+	s = satadd1(s, (int*)va, -1);
+	va_end(va);
+	return s;
+}
diff --git a/sys/src/libsat/satmore.c b/sys/src/libsat/satmore.c
new file mode 100644
index 000000000..7a316f1ab
--- /dev/null
+++ b/sys/src/libsat/satmore.c
@@ -0,0 +1,24 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+int
+satmore(SATSolve *s)
+{
+	int *a, i, n;
+
+	if(s == nil) return 1;
+	s->scrap = a = satrealloc(s, nil, s->nvar * sizeof(int));
+	n = 0;
+	for(i = 0; i < s->nvar; i++)
+		switch(s->lit[2*i].val){
+		case 0: a[n++] = i+1; break;
+		case 1: a[n++] = -(i+1); break;
+		}
+	if(n > 0)
+		satadd1(s, a, n);
+	free(a);
+	s->scrap = nil;
+	return satsolve(s);
+}
diff --git a/sys/src/libsat/satrange.c b/sys/src/libsat/satrange.c
new file mode 100644
index 000000000..76caa5713
--- /dev/null
+++ b/sys/src/libsat/satrange.c
@@ -0,0 +1,68 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+static SATSolve *
+satmin(SATSolve *s, int *a, int n, int *id, int *l, int m, int mul)
+{
+	int i;
+	
+	if(m > n) return s;
+	for(i = 0; i < m; i++)
+		id[i] = i;
+	for(;;){
+		for(i = 0; i < m; i++)
+			l[i] = a[id[i]] * mul;
+		s = satadd1(s, l, m);
+		for(i = m-1; i >= 0; i--){
+			if(++id[i] < n+i+1-m)
+				break;
+			if(i == 0)
+				return s;
+		}
+		while(++i < m)
+			id[i] = id[i-1]+1;
+	}
+}
+
+SATSolve *
+satrange1(SATSolve *s, int *a, int n, int min, int max)
+{
+	int sz, na;
+	
+	if(s == nil){
+		s = satnew();
+		if(s == nil)
+			saterror(nil, "satnew: %r");
+	}
+	if(n < 0)
+		for(n = 0; a[n] != 0; n++)
+			;
+	if(min > n || max < 0)
+		return sataddv(s, 0);
+	if(min < 0) min = 0;
+	if(max > n) max = n;
+	sz = n+1-min;
+	if(min == 0 || max != n && sz < max+1) sz = max+1;
+	if(s->cflclalloc < 2*sz){
+		na = -(-2*sz & -CFLCLALLOC);
+		s->cflcl = satrealloc(s, s->cflcl, na * sizeof(int));
+		s->cflclalloc = na;
+	}
+	s = satmin(s, a, n, s->cflcl, s->cflcl+sz, max+1, -1);
+	s = satmin(s, a, n, s->cflcl, s->cflcl+sz, n+1-min, 1);
+	return s;
+}
+
+SATSolve *
+satrangev(SATSolve *s, int min, int max, ...)
+{
+	va_list va;
+	
+	va_start(va, max);
+	/* horrible hack */
+	s = satrange1(s, (int*)va, -1, min, max);
+	va_end(va);
+	return s;
+}
diff --git a/sys/src/libsat/satsolve.c b/sys/src/libsat/satsolve.c
new file mode 100644
index 000000000..94dec8598
--- /dev/null
+++ b/sys/src/libsat/satsolve.c
@@ -0,0 +1,887 @@
+#include <u.h>
+#include <libc.h>
+#include <sat.h>
+#include "impl.h"
+
+/* the solver follows Algorithm C from Knuth's The Art of Computer Programming, Vol. 4, Fascicle 6 */
+
+#define verbosestate 0
+#define verboseforcing 0
+#define verboseconflict 0
+#define paranoia 0
+#define sanity(s) if(paranoia) satsanity(s)
+
+void
+sataddtrail(SATSolve *s, int l)
+{
+	s->trail[s->ntrail++] = l;
+	s->lit[l].val = 1;
+	s->lit[NOT(l)].val = 0;
+	s->var[VAR(l)].lvl = s->lvl;
+	s->agility -= s->agility >> 13;
+	if(((s->var[VAR(l)].flags ^ l) & 1) != 0)
+		s->agility += 1<<19;
+	if(verbosestate) satprintstate(s);
+}
+
+/* compute watchlists from scratch */
+static void
+rewatch(SATSolve *s)
+{
+	SATLit *l;
+	SATClause *c;
+	int i, j, x;
+	
+	for(l = s->lit; l < s->lit + 2*s->nvar; l++)
+		l->watch = nil;
+	for(c = s->cl; c != nil; c = c->next)
+		for(i = 0; i < 2; i++){
+			if(s->lit[c->l[i]].val == 0)
+				for(j = 2; j < c->n; j++)
+					if(s->lit[c->l[j]].val != 0){
+						x = c->l[i], c->l[i] = c->l[j], c->l[j] = x;
+						break;
+					}
+			c->watch[i] = s->lit[c->l[i]].watch;
+			s->lit[c->l[i]].watch = c;
+		}
+}
+
+/* jump back to decision level d */
+void
+satbackjump(SATSolve *s, int d)
+{
+	int l;
+	SATVar *v;
+
+	if(s->lvl == d) return;
+	while(s->ntrail > s->decbd[d + 1]){
+		l = s->trail[--s->ntrail];
+		v = &s->var[VAR(l)];
+		if((v->flags & VARUSER) != 0){ /* don't delete user assignments */
+			s->ntrail++;
+			break;
+		}
+		s->lit[l].val = -1;
+		s->lit[NOT(l)].val = -1;
+		v->flags = v->flags & ~1 | l & 1;
+		v->lvl = -1;
+		v->reason = nil;
+		v->isbinreason = 0;
+		if(v->heaploc < 0)
+			satheapput(s, v);
+	}
+	s->lvl = d;
+	if(s->forptr > s->ntrail) s->forptr = s->ntrail;
+	if(s->binptr > s->ntrail) s->binptr = s->ntrail;
+	if(verbosestate) satprintstate(s);
+}
+
+static void
+solvinit(SATSolve *s)
+{
+	satdebuginit(s);
+	satheapreset(s);
+	s->decbd = satrealloc(s, s->decbd, s->nvar * sizeof(int));
+	s->decbd[0] = 0;
+	s->trail = satrealloc(s, s->trail, sizeof(int) * s->nvar);
+	s->fullrlits = satrealloc(s, s->fullrlits, sizeof(int) * s->nvar);
+	s->lvlstamp = satrealloc(s, s->lvlstamp, sizeof(int) * s->nvar);
+	memset(s->lvlstamp, 0, sizeof(int) * s->nvar);
+	if(s->cflclalloc == 0){
+		s->cflcl = satrealloc(s, s->cflcl, CFLCLALLOC * sizeof(int));
+		s->cflclalloc = CFLCLALLOC;
+	}
+	rewatch(s);
+	
+	s->conflicts = 0;
+	s->nextpurge = s->purgeΔ;
+	s->purgeival = s->purgeΔ;
+	s->nextflush = 1;
+	s->flushu = 1;
+	s->flushv = 1;
+	s->flushθ = s->flushψ;
+	s->agility = 0;
+	
+	satbackjump(s, 0);
+	s->forptr = 0;
+	s->binptr = 0;
+}
+
+void
+satcleanup(SATSolve *s, int all)
+{
+	SATBlock *b, *bn;
+	
+	if(all){
+		*s->lastp[0] = nil;
+		s->learncl = nil;
+		s->lastp[1] = &s->learncl;
+		s->ncl = s->ncl0;
+	}
+	for(b = s->bl[1].next; b != &s->bl[1]; b = bn){
+		bn = b->next;
+		if(b->last != nil && !all) continue;
+		b->next->prev = b->prev;
+		b->prev->next = b->next;
+		free(b);
+	}
+	s->lastbl = s->bl[1].prev;
+	free(s->fullrlits);
+	s->fullrlits = nil;
+	free(s->lvlstamp);
+	s->lvlstamp = nil;
+	free(s->cflcl);
+	s->cflcl = nil;
+	s->cflclalloc = 0;
+}
+
+static void
+stampoverflow(SATSolve *s)
+{
+	int i;
+	
+	for(i = 0; i < s->nvar; i++){
+		s->var[i].stamp = 0;
+		s->lvlstamp[i] = 0;
+	}
+	s->stamp = -2;
+}
+
+/* "bump" the variable, i.e. increase its activity score. reduce all score when one exceeds MAXACTIVITY (1e100) */
+static void
+varbump(SATSolve *s, SATVar *v)
+{
+	v->activity += s->Δactivity;
+	satreheap(s, v);
+	if(v->activity < MAXACTIVITY) return;
+	for(v = s->var; v < s->var + s->nvar; v++)
+		if(v->activity != 0){
+			v->activity /= MAXACTIVITY;
+			if(v->activity < ε)
+				v->activity = ε;
+		}
+	s->Δactivity /= MAXACTIVITY;
+}
+
+/* ditto for clauses */
+static void
+clausebump(SATSolve *s, SATClause *c)
+{
+	c->activity += s->Δclactivity;
+	if(c->activity < MAXACTIVITY) return;
+	for(c = s->cl; c != nil; c = c->next)
+		if(c->activity != 0){
+			c->activity /= MAXACTIVITY;
+			if(c->activity < ε)
+				c->activity = ε;
+		}
+	s->Δclactivity /= MAXACTIVITY;
+}
+
+/* pick a literal. normally we pick the variable with highest activity from the heap. sometimes we goof and pick a random one. */
+static void
+decision(SATSolve *s)
+{
+	SATVar *v;
+
+	s->decbd[++s->lvl] = s->ntrail;
+	if((uint)s->randfn(s->randaux) < s->goofprob){
+		v = s->heap[satnrand(s, s->nheap)];
+		if(v->lvl < 0)
+			goto gotv;
+	}
+	do
+		v = satheaptake(s);
+	while(v->lvl >= 0);
+gotv:
+	sataddtrail(s, 2 * (v - s->var) + (v->flags & VARPHASE));
+}
+
+/* go through the watchlist of a literal that just turned out false. */
+/* full == 1 records the first conflict and goes on rather than aborting immediately */
+static SATClause *
+forcing(SATSolve *s, int l, int full)
+{
+	SATClause **cp, *rc, *c, *xp;
+	int v0;
+	int x, j;
+	
+	cp = &s->lit[l].watch;
+	rc = nil;
+	if(verboseforcing) print("forcing literal %d\n", signf(l));
+	while(c = *cp, c != nil){
+		if(l == c->l[0]){
+			/* this swap implies that the reason r for a literal l always has r->l[0]==l */
+			x = c->l[1], c->l[1] = c->l[0], c->l[0] = x;
+			xp = c->watch[1], c->watch[1] = c->watch[0], c->watch[0] = xp;
+		}
+		assert(c->l[1] == l);
+		v0 = s->lit[c->l[0]].val;
+		if(v0 > 0) /* the clause is true anyway */
+			goto next;
+		for(j = 2; j < c->n; j++)
+			if(s->lit[c->l[j]].val != 0){
+				/* found another literal to watch for this clause */
+				if(verboseforcing) print("moving clause %+Γ onto watchlist %d\n", c, signf(c->l[j]));
+				*cp = c->watch[1];
+				x = c->l[j], c->l[j] = c->l[1], c->l[1] = x;
+				c->watch[1] = s->lit[x].watch;
+				s->lit[x].watch = c;
+				goto cont;
+			}
+		if(v0 == 0){
+			/* conflict */
+			if(!full) return c;
+			if(rc == nil) rc = c;
+			goto next;
+		}
+		if(verboseforcing) print("inferring %d using clause %+Γ\n", signf(c->l[0]), c);
+		sataddtrail(s, c->l[0]);
+		s->var[VAR(c->l[0])].reason = c;
+	next:
+		cp = &c->watch[1];
+	cont: ;
+	}
+	return rc;
+}
+
+/* forcing() for binary implications */
+static uvlong
+binforcing(SATSolve *s, int l, int full)
+{
+	SATLit *lp;
+	int i, m;
+	uvlong rc;
+	
+	lp = &s->lit[l];
+	rc = 0;
+	if(verboseforcing && lp->nbimp > 0) print("forcing literal %d (binary)\n", signf(l));
+	for(i = 0; i < lp->nbimp; i++){
+		m = lp->bimp[i];
+		switch(s->lit[m].val){
+		case -1:
+			if(verboseforcing) print("inferring %d using binary clause (%d) ∨ %d\n", signf(m), -signf(l), signf(m));
+			sataddtrail(s, m);
+			s->var[VAR(m)].binreason = NOT(l);
+			s->var[VAR(m)].isbinreason = 1;
+			break;
+		case 0:
+			if(verboseforcing) print("conflict (%d) ∨ (%d)\n", -signf(l), signf(m));
+			if(rc == 0) rc = (uvlong)NOT(l) << 32 | (uint)m;
+			if(!full) return rc;
+			break;
+		}
+	}
+	return rc;
+}
+
+/* check if we can discard the previously learned clause because the current one subsumes it */
+static int
+checkdiscard(SATSolve *s)
+{
+	SATClause *c;
+	SATVar *v;
+	int q, j;
+	
+	if(s->lastp[1] == &s->learncl) return 0;
+	c = (SATClause*) ((uchar*) s->lastp[1] - (uchar*) &((SATClause*)0)->next);
+	if(s->lit[c->l[0]].val >= 0) return 0; /* clause is a reason, hands off */
+	q = s->ncflcl;
+	for(j = c->n - 1; q > 0 && j >= q; j--){
+		v = &s->var[VAR(c->l[j])];
+		/* check if literal is in the current clause */
+		if(c->l[j] == s->cflcl[0] || (uint)v->lvl <= s->cfllvl && v->stamp == s->stamp)
+			q--;
+	}
+	return q == 0;
+}
+
+/* add the clause we just learned to our collection */
+static SATClause *
+learn(SATSolve *s, int notriv)
+{
+	SATClause *r;
+	int i, l, triv;
+	
+	/* clauses that are too complicated are not worth it. learn the trivial clause (all decisions negated) instead */
+	if(triv = !notriv && s->ncflcl > s->lvl + s->trivlim){
+		assert(s->lvl + 1 <= s->cflclalloc);
+		for(i = 1; i <= s->lvl; i++)
+			s->cflcl[i] = NOT(s->trail[s->decbd[s->lvl + 1 - i]]);
+		s->ncflcl = s->lvl + 1;
+	}
+	if(s->ncflcl == 1) /* unit clauses are handled by putting them on the trail in conflict() */
+		return nil;
+	if(!triv && checkdiscard(s))
+		r = satreplclause(s, s->ncflcl);
+	else
+		r = satnewclause(s, s->ncflcl, 1);
+	r->n = s->ncflcl;
+	memcpy(r->l, s->cflcl, s->ncflcl * sizeof(int));
+	for(i = 0; i < 2; i++){
+		l = r->l[i];
+		r->watch[i] = s->lit[l].watch;
+		s->lit[l].watch = r;
+	}
+	return r;
+}
+
+/* recursive procedure to determine if a literal is redundant.
+ * to avoid repeated work, each known redundant literal is stamped with stamp+1
+ * and each known nonredundant literal is stamped with stamp+2.
+ */
+static int
+redundant(SATSolve *s, int l)
+{
+	SATVar *v, *w;
+	SATClause *c;
+	int i, r;
+	
+	v = &s->var[VAR(l)];
+	if(v->isbinreason){
+		/* stupid special case code */
+		r = v->binreason;
+		w = &s->var[VAR(r)];
+		if(w->lvl != 0){
+			if(w->stamp == s->stamp + 2)
+				return 0;
+			if(w->stamp < s->stamp && (s->lvlstamp[w->lvl] < s->stamp || !redundant(s, r))){
+				w->stamp = s->stamp + 2;
+				return 0;
+			}
+		}
+		v->stamp = s->stamp + 1;
+		return 1;
+	}
+	if(v->reason == nil) return 0; /* decision literals are never redundant */
+	c = v->reason;
+	for(i = 0; i < c->n; i++){
+		if(c->l[i] == NOT(l)) continue;
+		w = &s->var[VAR(c->l[i])];
+		if(w->lvl == 0)
+			continue; /* literals at level 0 are redundant */
+		if(w->stamp == s->stamp + 2)
+			return 0;
+		/* if the literal is not in the clause or known redundant, check if it is redundant */
+		/* we can skip the check if the level is not stamped: */
+		/* if there are no literals at the same level in the clause, it must be nonredundant */
+		if(w->stamp < s->stamp && (s->lvlstamp[w->lvl] < s->stamp || !redundant(s, c->l[i]))){
+			w->stamp = s->stamp + 2;
+			return 0;
+		}
+	}
+	v->stamp = s->stamp + 1;
+	return 1;
+}
+
+/* "blitting" a literal means to either add it to the conflict clause
+ * (if v->lvl < s->lvl) or to increment the counter of literals to be
+ * resolved, plus some bookkeeping.  */
+static void
+blit(SATSolve *s, int l)
+{
+	SATVar *v;
+	int p;
+	
+	v = &s->var[VAR(l)];
+	if(v->stamp == s->stamp) return;
+	v->stamp = s->stamp;
+	p = v->lvl;
+	if(p == 0) return;
+	if(verboseconflict) print("stamp %d %s (ctr=%d)\n", signf(l), p == s->lvl ? "and increment" : "and collect", s->cflctr);
+	varbump(s, v);
+	if(p == s->lvl){
+		s->cflctr++;
+		return;
+	}
+	if(s->ncflcl >= s->cflclalloc){
+		s->cflcl = satrealloc(s, s->cflcl, (s->cflclalloc + CFLCLALLOC) * sizeof(int));
+		s->cflclalloc += CFLCLALLOC;
+	}
+	s->cflcl[s->ncflcl++] = l;
+	if(p > s->cfllvl) s->cfllvl = p;
+	/* lvlstamp[p] == stamp if there is exactly one literal and ==stamp+1 if there is more than one literal on level p */
+	if(s->lvlstamp[p] <= s->stamp)
+		s->lvlstamp[p] = s->stamp + (s->lvlstamp[p] == s->stamp);
+}
+
+/* to resolve a conflict, we start with the conflict clause and use
+ * resolution (a ∨ b and ¬a ∨ c imply b ∨ c) with the reasons for the
+ * literals to remove all but one literal at the current level.  this
+ * gives a new "learned" clause with all literals false and we jump back
+ * to the second-highest level in it.  at this point, the clause implies
+ * the one remaining literal and we can continue.
+ * to do this quickly, rather than explicitly apply resolution, we keep a
+ * counter of literals at the highest level (unresolved literals) and an
+ * array with all other literals (which will become the learned clause). */
+static void
+conflict(SATSolve *s, SATClause *c, uvlong bin, int full)
+{
+	int i, j, l, p, *nl, found;
+	SATVar *v;
+	SATClause *r;
+
+	if(verboseconflict) satprintstate(s);
+	/* choose a new unique stamp value */
+	if(s->stamp >= (uint)-3)
+		stampoverflow(s);
+	s->stamp += 3;
+	s->ncflcl = 1;
+	s->cflctr = 0;
+	s->cfllvl = 0;
+	/* we start by blitting each literal in the conflict clause */
+	if(c != nil){
+		clausebump(s, c);
+		for(i = 0; i < c->n; i++)
+			blit(s, c->l[i]);
+		/* if there is only one literal l at the current level, we should have inferred ¬l at a lower level (bug). */
+		if(s->cflctr <= 1){
+			satprintstate(s);
+			print("conflict clause %+Γ\n", c);
+			assert(s->cflctr > 1);
+		}
+	}else{
+		blit(s, bin);
+		blit(s, bin>>32);
+		if(s->cflctr <= 1){
+			satprintstate(s);
+			print("binary conflict clause %d ∨ %d\n", (int)(bin>>32), (int)bin);
+			assert(s->cflctr > 1);
+		}
+	}
+	/* now we go backwards through the trail, decrementing the unresolved literal counter at each stamped literal */
+	/* and blitting the literals in their reason */
+	for(i = s->ntrail; --i >= 0; ){
+		v = &s->var[VAR(s->trail[i])];
+		if(v->stamp != s->stamp) continue;
+		if(verboseconflict) print("trail literal %d\n", signf(s->trail[i]));
+		if(--s->cflctr == 0) break;
+		if(v->isbinreason)
+			blit(s, v->binreason);
+		else if((r = v->reason) != nil){
+			clausebump(s, r);
+			for(j = 0; j < r->n; j++)
+				blit(s, r->l[j]);
+		}
+	}
+	/* i should point to the one remaining literal at the current level */
+	assert(i >= 0);
+	nl = s->cflcl;
+	nl[0] = NOT(s->trail[i]);
+	found = 0;
+	/* delete redundant literals. note we must watch a literal at cfllvl, so put it in l[1]. */
+	for(i = 1, j = 1; i < s->ncflcl; i++){
+		l = nl[i];
+		p = s->var[VAR(nl[i])].lvl;
+		/* lvlstamp[p] != s->stamp + 1 => only one literal at level p => must be nonredundant */
+		if(s->lvlstamp[p] != s->stamp + 1 || !redundant(s, l))
+			if(found || p < s->cfllvl)
+				nl[j++] = nl[i];
+			else{
+				/* watch this literal */
+				l = nl[i], nl[j++] = nl[1], nl[1] = l;
+				found = 1;
+			}
+	}
+	s->ncflcl = j;
+	if(!full){
+		/* normal mode: jump back and add to trail right away */
+		satbackjump(s, s->cfllvl);
+		sataddtrail(s, nl[0]);
+	}else{
+		/* purging: record minimum cfllvl and literals at that level */
+		if(s->cfllvl < s->fullrlvl){
+			s->fullrlvl = s->cfllvl;
+			s->nfullrlits = 0;
+		}
+		s->fullrlits[s->nfullrlits++] = nl[0];
+	}
+	r = learn(s, full);
+	if(!full && r != nil)
+		s->var[VAR(nl[0])].reason = r;
+	if(verboseconflict)
+		if(r != nil)
+			print("learned %+Γ\n", r);
+		else
+			print("learned %d\n", signf(nl[0]));
+	s->Δactivity *= s->varρ;
+	s->Δclactivity *= s->clauseρ;
+	s->conflicts++;
+}
+
+/* to purge, we need a fullrun that assigns values to all variables.
+ * during this we record the first conflict at each level, to be resolved
+ * later.  otherwise this is just a copy of the main loop which never
+ * purges or flushes. */
+static int
+fullrun(SATSolve *s)
+{
+	int l;
+	uvlong b;
+	SATClause *c;
+
+	while(s->ntrail < s->nvar){
+		decision(s);
+	re:
+		while(s->binptr < s->ntrail){
+			l = s->trail[s->binptr++];
+			b = binforcing(s, l, 1);
+			if(b != 0){
+				if(s->lvl == 0){
+					s->unsat = 1;
+					return -1;
+				}
+				if(s->nfullrcfl == 0 || s->lvl > CFLLVL(s->fullrcfl[s->nfullrcfl-1])){
+					s->fullrcfl = satrealloc(s, s->fullrcfl, sizeof(SATConflict) * (s->nfullrcfl + 1));
+					s->fullrcfl[s->nfullrcfl].lvl = 1<<31 | s->lvl;
+					s->fullrcfl[s->nfullrcfl++].b = b;
+				}
+			}
+			sanity(s);
+		}
+		while(s->forptr < s->ntrail){
+			l = s->trail[s->forptr++];
+			c = forcing(s, NOT(l), 1);
+			if(c != nil){
+				if(s->lvl == 0){
+					s->unsat = 1;
+					return -1;
+				}
+				if(s->nfullrcfl == 0 || s->lvl > CFLLVL(s->fullrcfl[s->nfullrcfl-1])){
+					s->fullrcfl = satrealloc(s, s->fullrcfl, sizeof(SATConflict) * (s->nfullrcfl + 1));
+					s->fullrcfl[s->nfullrcfl].lvl = s->lvl;
+					s->fullrcfl[s->nfullrcfl++].c = c;
+				}
+			}
+		}
+		if(s->binptr < s->ntrail) goto re;
+	}
+	return 0;
+}
+
+/* assign range scores to all clauses.
+ * p == number of levels that have positive literals in the clause.
+ * r == number of levels that have literals in the clause.
+ * range == min(floor(16 * (p + α (r - p))), 255) with magic constant α. */
+static void
+ranges(SATSolve *s)
+{
+	SATClause *c;
+	int p, r, k, l, v;
+	uint ci;
+
+	ci = 2;
+	memset(s->lvlstamp, 0, sizeof(int) * s->nvar);
+	memset(s->rangecnt, 0, sizeof(s->rangecnt));
+	for(c = s->learncl; c != nil; c = c->next, ci += 2){
+		if(!s->var[VAR(c->l[0])].binreason && s->var[VAR(c->l[0])].reason == c){
+			c->range = 0;
+			s->rangecnt[0]++;
+			continue;
+		}
+		p = 0;
+		r = 0;
+		for(k = 0; k < c->n; k++){
+			l = c->l[k];
+			v = s->var[VAR(l)].lvl;
+			if(v == 0){
+				if(s->lit[l].val == 1){
+					c->range = 256;
+					goto next;
+				}
+			}else{
+				if(s->lvlstamp[v] < ci){
+					s->lvlstamp[v] = ci;
+					r++;
+				}
+				if(s->lvlstamp[v] == ci && s->lit[l].val == 1){
+					s->lvlstamp[v] = ci + 1;
+					p++;
+				}
+			}
+		}
+		r = 16 * (p + s->purgeα * (r - p));
+		if(r > 255) r = 255;
+		c->range = r;
+		s->rangecnt[r]++;
+	next: ;
+	}
+}
+
+/* resolve conflicts found during fullrun() */
+static void
+fullrconflicts(SATSolve *s)
+{
+	SATConflict *cfl;
+	int i;
+	
+	s->fullrlvl = s->lvl;
+	s->nfullrlits = 0;
+	for(cfl = &s->fullrcfl[s->nfullrcfl - 1]; cfl >= s->fullrcfl; cfl--){
+		satbackjump(s, CFLLVL(*cfl));
+		if(cfl->lvl < 0)
+			conflict(s, nil, cfl->b, 1);
+		else
+			conflict(s, cfl->c, 0, 1);
+	}
+	satbackjump(s, 0);
+	if(s->fullrlvl == 0)
+		for(i = 0; i < s->nfullrlits; i++)
+			sataddtrail(s, s->fullrlits[i]);
+	free(s->fullrcfl);
+	s->fullrcfl = nil;
+}
+
+/* note that nil > *, this simplifies the algorithm by having nil "bubble" to the top */
+static int
+actgt(SATClause *a, SATClause *b)
+{
+	if(b == nil) return 0;
+	if(a == nil) return 1;
+	return a->activity > b->activity || a->activity == b->activity && a > b;
+}
+
+/* select n clauses to keep
+ * first we find the upper limit j on the range score
+ * to get the exact number, we move htot clauses from j to j+1
+ * to this end, we put them in a max-heap of size htot, sorted by activity,
+ * continually replacing the largest element if we find a less active clause.
+ * the heap starts out filled with nil and the nil are replaced during the first
+ * htot iterations. */
+#define LEFT(i) (2*(i)+1)
+#define RIGHT(i) (2*(i)+2)
+static int
+judgement(SATSolve *s, int n)
+{
+	int cnt, i, j, htot, m;
+	SATClause *c, **h, *z;
+	
+	cnt = 0;
+	for(j = 0; j < 256; j++){
+		cnt += s->rangecnt[j];
+		if(cnt >= n) break;
+	}
+	if(j == 256) return j;
+	if(cnt > n){
+		htot = cnt - n;
+		h = satrealloc(s, nil, sizeof(SATClause *) * htot);
+		memset(h, 0, sizeof(SATClause *) * htot);
+		for(c = s->learncl; c != nil; c = c->next){
+			if(c->range != j || actgt(c, h[0])) continue;
+			h[0] = c;
+			m = 0;
+			for(;;){
+				i = m;
+				if(LEFT(i) < htot && actgt(h[LEFT(i)], h[m])) m = LEFT(i);
+				if(RIGHT(i) < htot && actgt(h[RIGHT(i)], h[m])) m = RIGHT(i);
+				if(i == m) break;
+				z = h[i], h[i] = h[m], h[m] = z;
+			}
+		}
+		for(i = 0; i < htot; i++)
+			if(h[i] != nil)
+				h[i]->range = j + 1;
+		free(h);
+	}
+	return j;
+}
+
+/* during purging we remove permanently false literals from learned clauses.
+ * returns 1 if the clause can be deleted entirely. */
+static int
+cleanupclause(SATSolve *s, SATClause *c)
+{
+	int i, k;
+	
+	for(i = 0; i < c->n; i++)
+		if(s->lit[c->l[i]].val == 0)
+			break;
+	if(i == c->n) return 0;
+	for(k = i; i < c->n; i++)
+		if(s->lit[c->l[i]].val != 0)
+			c->l[k++] = c->l[i];
+	c->n = k;
+	if(k > 1) return 0;
+	if(k == 0)
+		s->unsat = 1;
+	else if(s->lit[c->l[0]].val < 0)
+		sataddtrail(s, c->l[0]);
+	return 1;
+}
+
+/* delete clauses by overwriting them. don't delete empty blocks; we're going to fill them up soon enough again. */
+static void
+execution(SATSolve *s, int j)
+{
+	SATClause *c, *n, **cp, *p;
+	SATBlock *b;
+	SATVar *v0;
+	int f, sz;
+
+	b = s->bl[1].next;
+	p = (SATClause*) b->data;
+	s->ncl = s->ncl0;
+	cp = &s->learncl;
+	for(c = p; c != nil; c = n){
+		n = c->next;
+		if(c->range > j || cleanupclause(s, c))
+			continue;
+		sz = sizeof(SATClause) + (c->n - 1) * sizeof(int);
+		f = (uchar*)b + SATBLOCKSZ - (uchar*)p;
+		if(f < sz){
+			memset(p, 0, f);
+			b = b->next;
+			assert(b != &s->bl[1]);
+			p = (SATClause *) b->data;
+		}
+		b->last = p;
+		/* update reason field of the first variable (if applicable) */
+		v0 = &s->var[VAR(c->l[0])];
+		if(!v0->isbinreason && v0->reason == c)
+			v0->reason = p;
+		memmove(p, c, sz);
+		*cp = p;
+		cp = &p->next;
+		p = (void*)((uintptr)p + sz + CLAUSEALIGN - 1 & -CLAUSEALIGN);
+		b->end = p;
+		s->ncl++;
+	}
+	*cp = nil;
+	*s->lastp[0] = s->learncl;
+	s->lastp[1] = cp;
+	s->lastbl = b;
+	f = (uchar*)b + SATBLOCKSZ - (uchar*)p;
+	memset(p, 0, f);
+	for(b = b->next; b != &s->bl[1]; b = b->next){
+		b->last = nil;
+		b->end = b->data;
+	}
+}
+
+static void
+thepurge(SATSolve *s)
+{
+	int nkeep, i, j;
+	SATVar *v;
+	
+	s->purgeival += s->purgeδ;
+	s->nextpurge = s->conflicts + s->purgeival;
+	nkeep = (s->ncl - s->ncl0) / 2;
+	for(i = 0; i < s->ntrail; i++){
+		v = &s->var[VAR(s->trail[i])];
+		if(!v->isbinreason && v->reason != nil)
+			nkeep++;
+	}
+	if(nkeep <= 0) return; /* shouldn't happen */
+	s->nfullrcfl = 0;
+	if(fullrun(s) < 0){ /* accidentally determined UNSAT during fullrun() */
+		free(s->fullrcfl);
+		s->fullrcfl = nil;
+		return;
+	}
+	ranges(s);
+	fullrconflicts(s);
+	j = judgement(s, nkeep);
+	execution(s, j);
+	rewatch(s);
+}
+
+/* to avoid getting stuck, flushing backs up the trail to remove low activity variables.
+ * don't worry about throwing out high activity ones, they'll get readded quickly. */
+static void
+theflush(SATSolve *s)
+{
+	double actk;
+	int dd, l;
+
+	/* "reluctant doubling" wizardry to determine when to flush */
+	if((s->flushu & -s->flushu) == s->flushv){
+		s->flushu++;
+		s->flushv = 1;
+		s->flushθ = s->flushψ;
+	}else{
+		s->flushv *= 2;
+		s->flushθ += s->flushθ >> 4;
+	}
+	s->nextflush = s->conflicts + s->flushv;
+	if(s->agility > s->flushθ) return; /* don't flush when we're too busy */
+	/* clean up the heap so that a free variable is at the top */
+	while(s->nheap > 0 && s->heap[0]->lvl >= 0)
+		satheaptake(s);
+	if(s->nheap == 0) return; /* shouldn't happen */
+	actk = s->heap[0]->activity;
+	for(dd = 0; dd < s->lvl; dd++){
+		l = s->trail[s->decbd[dd+1]];
+		if(s->var[VAR(l)].activity < actk)
+			break;
+	}
+	satbackjump(s, dd);
+}
+
+int
+satsolve(SATSolve *s)
+{
+	int l;
+	SATClause *c;
+	uvlong b;
+
+	if(s == nil) return 1;
+	if(s->scratched) return -1;
+	if(s->nvar == 0) return 1;
+	solvinit(s);
+	
+	while(!s->unsat){
+	re:
+		while(s->binptr < s->ntrail){
+			l = s->trail[s->binptr++];
+			b = binforcing(s, l, 0);
+			sanity(s);
+			if(b != 0){
+				if(s->lvl == 0) goto unsat;
+				conflict(s, nil, b, 0);
+				sanity(s);
+			}
+		}
+		while(s->forptr < s->ntrail){
+			l = s->trail[s->forptr++];
+			c = forcing(s, NOT(l), 0);
+			sanity(s);
+			if(c != nil){
+				if(s->lvl == 0) goto unsat;
+				conflict(s, c, 0, 0);
+				sanity(s);
+			}
+		}
+		if(s->binptr < s->ntrail) goto re;
+		if(s->ntrail == s->nvar) goto out;
+		if(s->conflicts >= s->nextpurge)
+			thepurge(s);
+		else if(s->conflicts >= s->nextflush)
+			theflush(s);
+		else
+			decision(s);
+	}
+unsat:
+	s->unsat = 1;
+out:
+	satcleanup(s, 0);
+	return !s->unsat;
+}
+
+void
+satreset(SATSolve *s)
+{
+	int i;
+
+	if(s == nil || s->decbd == nil) return;
+	satbackjump(s, -1);
+	s->lvl = 0;
+	for(i = 0; i < s->nvar; i++){
+		s->var[i].activity = 0;
+		s->var[i].flags |= VARPHASE;
+	}
+	satcleanup(s, 1);
+	s->Δactivity = 1;
+	s->Δclactivity = 1;
+}
diff --git a/sys/src/mkfile b/sys/src/mkfile
index 683011fa8..99bb4a89a 100644
--- a/sys/src/mkfile
+++ b/sys/src/mkfile
@@ -30,6 +30,7 @@ LIBS=\
 	libndb\
 	libplumb\
 	libregexp\
+	libsat\
 	libscribble\
 	libsec\
 	libstdio\