Add utf8 string view

std/unicode.zig

@@ -1,4 +1,5 @@
 const std = @import("./index.zig");
+const debug = std.debug;
 
 /// Given the first byte of a UTF-8 codepoint,
 /// returns a number 1-4 indicating the total length of the codepoint in bytes.
@@ -25,8 +26,8 @@ pub fn utf8Decode(bytes: []const u8) !u32 {
     };
 }
 pub fn utf8Decode2(bytes: []const u8) !u32 {
-    std.debug.assert(bytes.len == 2);
-    std.debug.assert(bytes[0] & 0b11100000 == 0b11000000);
+    debug.assert(bytes.len == 2);
+    debug.assert(bytes[0] & 0b11100000 == 0b11000000);
     var value: u32 = bytes[0] & 0b00011111;
 
     if (bytes[1] & 0b11000000 != 0b10000000) return error.Utf8ExpectedContinuation;
@@ -38,8 +39,8 @@ pub fn utf8Decode2(bytes: []const u8) !u32 {
     return value;
 }
 pub fn utf8Decode3(bytes: []const u8) !u32 {
-    std.debug.assert(bytes.len == 3);
-    std.debug.assert(bytes[0] & 0b11110000 == 0b11100000);
+    debug.assert(bytes.len == 3);
+    debug.assert(bytes[0] & 0b11110000 == 0b11100000);
     var value: u32 = bytes[0] & 0b00001111;
 
     if (bytes[1] & 0b11000000 != 0b10000000) return error.Utf8ExpectedContinuation;
@@ -56,8 +57,8 @@ pub fn utf8Decode3(bytes: []const u8) !u32 {
     return value;
 }
 pub fn utf8Decode4(bytes: []const u8) !u32 {
-    std.debug.assert(bytes.len == 4);
-    std.debug.assert(bytes[0] & 0b11111000 == 0b11110000);
+    debug.assert(bytes.len == 4);
+    debug.assert(bytes[0] & 0b11111000 == 0b11110000);
     var value: u32 = bytes[0] & 0b00000111;
 
     if (bytes[1] & 0b11000000 != 0b10000000) return error.Utf8ExpectedContinuation;
@@ -78,6 +79,136 @@ pub fn utf8Decode4(bytes: []const u8) !u32 {
     return value;
 }
 
+pub fn utf8ValidateSlice(s: []const u8) bool {
+    var i: usize = 0;
+    while (i < s.len) {
+        if (utf8ByteSequenceLength(s[i])) |cp_len| {
+            if (i + cp_len > s.len) {
+                return false;
+            }
+
+            if (utf8Decode(s[i..i+cp_len])) |_| {} else |_| { return false; }
+            i += cp_len;
+        } else |err| {
+            return false;
+        }
+    }
+    return true;
+}
+
+const Utf8View = struct {
+    bytes: []const u8,
+
+    pub fn init(s: []const u8) !Utf8View {
+        if (!utf8ValidateSlice(s)) {
+            return error.InvalidUtf8;
+        }
+
+        return initUnchecked(s);
+    }
+
+    pub fn initUnchecked(s: []const u8) Utf8View {
+        return Utf8View {
+            .bytes = s,
+        };
+    }
+
+    pub fn initComptime(comptime s: []const u8) Utf8View {
+        if (comptime init(s)) |r| {
+            return r;
+        } else |err| switch (err) {
+            error.InvalidUtf8 => {
+                @compileError("invalid utf8");
+                unreachable;
+            }
+        }
+    }
+
+    pub fn Iterator(s: &const Utf8View) Utf8Iterator {
+        return Utf8Iterator {
+            .bytes = s.bytes,
+            .i = 0,
+        };
+    }
+};
+
+const Utf8Iterator = struct {
+    bytes: []const u8,
+    i: usize,
+
+    pub fn nextCodepointSlice(it: &Utf8Iterator) ?[]const u8 {
+        if (it.i >= it.bytes.len) {
+            return null;
+        }
+
+        const cp_len = utf8ByteSequenceLength(it.bytes[it.i]) catch unreachable;
+
+        it.i += cp_len;
+        return it.bytes[it.i-cp_len..it.i];
+    }
+
+    pub fn nextCodepoint(it: &Utf8Iterator) ?u32 {
+        const slice = it.nextCodepointSlice() ?? return null;
+
+        const r = switch (slice.len) {
+            1 => u32(slice[0]),
+            2 => utf8Decode2(slice),
+            3 => utf8Decode3(slice),
+            4 => utf8Decode4(slice),
+            else => unreachable,
+        };
+
+        return r catch unreachable;
+    }
+};
+
+test "utf8 iterator on ascii" {
+    const s = Utf8View.initComptime("abc");
+
+    var it1 = s.Iterator();
+    debug.assert(std.mem.eql(u8, "a", ??it1.nextCodepointSlice()));
+    debug.assert(std.mem.eql(u8, "b", ??it1.nextCodepointSlice()));
+    debug.assert(std.mem.eql(u8, "c", ??it1.nextCodepointSlice()));
+    debug.assert(it1.nextCodepointSlice() == null);
+
+    var it2 = s.Iterator();
+    debug.assert(??it2.nextCodepoint() == 'a');
+    debug.assert(??it2.nextCodepoint() == 'b');
+    debug.assert(??it2.nextCodepoint() == 'c');
+    debug.assert(it2.nextCodepoint() == null);
+}
+
+test "utf8 view bad" {
+    // Compile-time error.
+    // const s3 = Utf8View.initComptime("\xfe\xf2");
+
+    const s = Utf8View.init("hel\xadlo");
+    if (s) |_| { unreachable; } else |err| { debug.assert(err == error.InvalidUtf8); }
+}
+
+test "utf8 view ok" {
+    const s = Utf8View.initComptime("東京市");
+
+    var it1 = s.Iterator();
+    debug.assert(std.mem.eql(u8, "東", ??it1.nextCodepointSlice()));
+    debug.assert(std.mem.eql(u8, "京", ??it1.nextCodepointSlice()));
+    debug.assert(std.mem.eql(u8, "市", ??it1.nextCodepointSlice()));
+    debug.assert(it1.nextCodepointSlice() == null);
+
+    var it2 = s.Iterator();
+    debug.assert(??it2.nextCodepoint() == 0x6771);
+    debug.assert(??it2.nextCodepoint() == 0x4eac);
+    debug.assert(??it2.nextCodepoint() == 0x5e02);
+    debug.assert(it2.nextCodepoint() == null);
+}
+
+test "bad utf8 slice" {
+    debug.assert(utf8ValidateSlice("abc"));
+    debug.assert(!utf8ValidateSlice("abc\xc0"));
+    debug.assert(!utf8ValidateSlice("abc\xc0abc"));
+    debug.assert(utf8ValidateSlice("abc\xdf\xbf"));
+}
+
 test "valid utf8" {
     testValid("\x00", 0x0);
     testValid("\x20", 0x20);
@@ -145,17 +276,17 @@ fn testError(bytes: []const u8, expected_err: error) void {
     if (testDecode(bytes)) |_| {
         unreachable;
     } else |err| {
-        std.debug.assert(err == expected_err);
+        debug.assert(err == expected_err);
     }
 }
 
 fn testValid(bytes: []const u8, expected_codepoint: u32) void {
-    std.debug.assert((testDecode(bytes) catch unreachable) == expected_codepoint);
+    debug.assert((testDecode(bytes) catch unreachable) == expected_codepoint);
 }
 
 fn testDecode(bytes: []const u8) !u32 {
     const length = try utf8ByteSequenceLength(bytes[0]);
     if (bytes.len < length) return error.UnexpectedEof;
-    std.debug.assert(bytes.len == length);
+    debug.assert(bytes.len == length);
     return utf8Decode(bytes);
 }