const std = @import("std.zig");
const debug = std.debug;
const mem = std.mem;
const Allocator = mem.Allocator;
const assert = debug.assert;
const testing = std.testing;
const ArrayList = std.ArrayList;

/// A buffer that allocates memory and maintains a null byte at the end.
pub const Buffer = struct {
    list: ArrayList(u8),

    /// Must deinitialize with deinit.
    pub fn init(allocator: *Allocator, m: []const u8) !Buffer {
        var self = try initSize(allocator, m.len);
        mem.copy(u8, self.list.items, m);
        return self;
    }

    /// Initialize memory to size bytes of undefined values.
    /// Must deinitialize with deinit.
    pub fn initSize(allocator: *Allocator, size: usize) !Buffer {
        var self = initNull(allocator);
        try self.resize(size);
        return self;
    }

    /// Initialize with capacity to hold at least num bytes.
    /// Must deinitialize with deinit.
    pub fn initCapacity(allocator: *Allocator, num: usize) !Buffer {
        var self = Buffer{ .list = try ArrayList(u8).initCapacity(allocator, num + 1) };
        self.list.appendAssumeCapacity(0);
        return self;
    }

    /// Must deinitialize with deinit.
    /// None of the other operations are valid until you do one of these:
    /// * ::replaceContents
    /// * ::resize
    pub fn initNull(allocator: *Allocator) Buffer {
        return Buffer{ .list = ArrayList(u8).init(allocator) };
    }

    /// Must deinitialize with deinit.
    pub fn initFromBuffer(buffer: Buffer) !Buffer {
        return Buffer.init(buffer.list.allocator, buffer.toSliceConst());
    }

    /// Buffer takes ownership of the passed in slice. The slice must have been
    /// allocated with `allocator`.
    /// Must deinitialize with deinit.
    pub fn fromOwnedSlice(allocator: *Allocator, slice: []u8) !Buffer {
        var self = Buffer{ .list = ArrayList(u8).fromOwnedSlice(allocator, slice) };
        try self.list.append(0);
        return self;
    }

    /// The caller owns the returned memory. The Buffer becomes null and
    /// is safe to `deinit`.
    pub fn toOwnedSlice(self: *Buffer) [:0]u8 {
        const allocator = self.list.allocator;
        const result = self.list.toOwnedSlice();
        self.* = initNull(allocator);
        return result[0 .. result.len - 1 :0];
    }

    pub fn allocPrint(allocator: *Allocator, comptime format: []const u8, args: var) !Buffer {
        const size = std.math.cast(usize, std.fmt.count(format, args)) catch |err| switch (err) {
            error.Overflow => return error.OutOfMemory,
        };
        var self = try Buffer.initSize(allocator, size);
        assert((std.fmt.bufPrint(self.list.items, format, args) catch unreachable).len == size);
        return self;
    }

    pub fn deinit(self: *Buffer) void {
        self.list.deinit();
    }

    pub fn span(self: var) @TypeOf(self.list.items[0 .. self.list.len - 1 :0]) {
        return self.list.span()[0..self.len() :0];
    }

    /// Deprecated: use `span`
    pub fn toSlice(self: Buffer) [:0]u8 {
        return self.span();
    }

    /// Deprecated: use `span`
    pub fn toSliceConst(self: Buffer) [:0]const u8 {
        return self.span();
    }

    pub fn shrink(self: *Buffer, new_len: usize) void {
        assert(new_len <= self.len());
        self.list.shrink(new_len + 1);
        self.list.items[self.len()] = 0;
    }

    pub fn resize(self: *Buffer, new_len: usize) !void {
        try self.list.resize(new_len + 1);
        self.list.items[self.len()] = 0;
    }

    pub fn isNull(self: Buffer) bool {
        return self.list.len == 0;
    }

    pub fn len(self: Buffer) usize {
        return self.list.len - 1;
    }

    pub fn capacity(self: Buffer) usize {
        return if (self.list.items.len > 0)
            self.list.items.len - 1
        else
            0;
    }

    pub fn append(self: *Buffer, m: []const u8) !void {
        const old_len = self.len();
        try self.resize(old_len + m.len);
        mem.copy(u8, self.list.toSlice()[old_len..], m);
    }

    pub fn appendByte(self: *Buffer, byte: u8) !void {
        const old_len = self.len();
        try self.resize(old_len + 1);
        self.list.toSlice()[old_len] = byte;
    }

    pub fn eql(self: Buffer, m: []const u8) bool {
        return mem.eql(u8, self.toSliceConst(), m);
    }

    pub fn startsWith(self: Buffer, m: []const u8) bool {
        if (self.len() < m.len) return false;
        return mem.eql(u8, self.list.items[0..m.len], m);
    }

    pub fn endsWith(self: Buffer, m: []const u8) bool {
        const l = self.len();
        if (l < m.len) return false;
        const start = l - m.len;
        return mem.eql(u8, self.list.items[start..l], m);
    }

    pub fn replaceContents(self: *Buffer, m: []const u8) !void {
        try self.resize(m.len);
        mem.copy(u8, self.list.toSlice(), m);
    }

    pub fn outStream(self: *Buffer) std.io.OutStream(*Buffer, error{OutOfMemory}, appendWrite) {
        return .{ .context = self };
    }

    /// Same as `append` except it returns the number of bytes written, which is always the same
    /// as `m.len`. The purpose of this function existing is to match `std.io.OutStream` API.
    pub fn appendWrite(self: *Buffer, m: []const u8) !usize {
        try self.append(m);
        return m.len;
    }
};

test "simple Buffer" {
    var buf = try Buffer.init(testing.allocator, "");
    defer buf.deinit();

    testing.expect(buf.len() == 0);
    try buf.append("hello");
    try buf.append(" ");
    try buf.append("world");
    testing.expect(buf.eql("hello world"));
    testing.expect(mem.eql(u8, mem.toSliceConst(u8, buf.toSliceConst().ptr), buf.toSliceConst()));

    var buf2 = try Buffer.initFromBuffer(buf);
    defer buf2.deinit();
    testing.expect(buf.eql(buf2.toSliceConst()));

    testing.expect(buf.startsWith("hell"));
    testing.expect(buf.endsWith("orld"));

    try buf2.resize(4);
    testing.expect(buf.startsWith(buf2.toSlice()));
}

test "Buffer.initSize" {
    var buf = try Buffer.initSize(testing.allocator, 3);
    defer buf.deinit();
    testing.expect(buf.len() == 3);
    try buf.append("hello");
    testing.expect(mem.eql(u8, buf.toSliceConst()[3..], "hello"));
}

test "Buffer.initCapacity" {
    var buf = try Buffer.initCapacity(testing.allocator, 10);
    defer buf.deinit();
    testing.expect(buf.len() == 0);
    testing.expect(buf.capacity() >= 10);
    const old_cap = buf.capacity();
    try buf.append("hello");
    testing.expect(buf.len() == 5);
    testing.expect(buf.capacity() == old_cap);
    testing.expect(mem.eql(u8, buf.toSliceConst(), "hello"));
}

test "Buffer.print" {
    var buf = try Buffer.init(testing.allocator, "");
    defer buf.deinit();

    try buf.outStream().print("Hello {} the {}", .{ 2, "world" });
    testing.expect(buf.eql("Hello 2 the world"));
}

test "Buffer.outStream" {
    var buffer = try Buffer.initSize(testing.allocator, 0);
    defer buffer.deinit();
    const buf_stream = buffer.outStream();

    const x: i32 = 42;
    const y: i32 = 1234;
    try buf_stream.print("x: {}\ny: {}\n", .{ x, y });

    testing.expect(mem.eql(u8, buffer.toSlice(), "x: 42\ny: 1234\n"));
}