const std = @import("std");
const assert = std.debug.assert;

pub const Var = struct {
    key: []const u8,
    val: []const u8,

    pub fn format(self: Var, w: *std.io.Writer) !void {
        const spacebuf = [_]u8{' '} ** 32;
        const equal_sign = " = ";

        try w.writeAll(self.key);
        try w.writeAll(equal_sign);
        if (std.mem.indexOfScalar(u8, self.val, '\n')) |eol0| {
            try w.writeAll(self.val[0 .. eol0 + 1]);
            var val = self.val[eol0 + 1 ..];
            while (val.len > 0) {
                const eol = std.mem.indexOfScalar(u8, val, '\n') orelse val.len - 1;
                const line = val[0 .. eol + 1];
                var spaces = self.key.len;
                while (spaces > 0) {
                    const n = @min(spaces, spacebuf.len);
                    try w.writeAll(spacebuf[0..n]);
                    spaces -= n;
                }
                try w.writeAll(equal_sign);
                try w.writeAll(line);
                val = val[eol + 1 ..];
            }
        } else {
            try w.writeAll(self.val);
        }
    }
};

const Var_Header = packed struct(u64) {
    key_len: u32,
    val_len: u32,

    fn encode(self: Var_Header) [8]u8 {
        var header: u64 = @intCast(self.key_len);
        header <<= 32;
        header |= @intCast(self.val_len);
        return std.mem.toBytes(std.mem.nativeToBig(u64, header));
    }

    fn decode(buf: []const u8) Var_Header {
        const header = std.mem.bigToNative(u64, std.mem.bytesToValue(u64, buf[0..8]));
        return .{
            .key_len = @intCast(header >> 32),
            .val_len = @intCast(header & 0xffffffff),
        };
    }
};

pub const Iterator = struct {
    rest: []const u8,

    pub fn next(self: *Iterator) ?Var {
        if (self.rest.len == 0) return null;

        const header = Var_Header.decode(self.rest);
        const v: Var = .{
            .key = self.rest[@sizeOf(Var_Header) .. @sizeOf(Var_Header) + header.key_len],
            .val = self.rest[@sizeOf(Var_Header) + header.key_len .. @sizeOf(Var_Header) + header.key_len + header.val_len],
        };
        self.rest = self.rest[@sizeOf(Var_Header) + v.key.len + v.val.len ..];
        return v;
    }
};

pub const Parse_Result = union(enum) {
    conf: Config,
    err: struct {
        msg: []const u8,
        line: usize,
    },
};

pub const Config = struct {
    allocator: std.mem.Allocator,
    buf: []u8, // encoded key/value pairs
    len: usize, // used length of `buf`

    pub fn init(allocator: std.mem.Allocator) Config {
        return .{ .allocator = allocator, .buf = &.{}, .len = 0 };
    }

    pub fn from_file(allocator: std.mem.Allocator, path: []const u8) !Parse_Result {
        var file = std.fs.openFileAbsolute(path, .{}) catch |err| {
            return switch (err) {
                error.FileNotFound => .{ .conf = .init(allocator) },
                else => err,
            };
        };
        defer file.close();

        var read_buf: [1024]u8 = undefined;
        var reader = file.reader(&read_buf);
        return parse(allocator, &reader.interface, read_buf.len);
    }

    fn parse(allocator: std.mem.Allocator, reader: *std.io.Reader, comptime max_line_len: usize) !Parse_Result {
        const spaces = " \t\r";

        var allocating: std.io.Writer.Allocating = .init(allocator);

        var header: Var_Header = undefined;
        var header_off: usize = 0;
        var need_key = true;
        var file_line: usize = 0;
        while (true) {
            file_line += 1;
            const raw_line = reader.takeDelimiterExclusive('\n') catch |err| switch (err) {
                error.StreamTooLong => {
                    return .{
                        .err = .{
                            .msg = std.fmt.comptimePrint("key/value line exceeds the maximum of {} bytes", .{max_line_len}),
                            .line = file_line,
                        },
                    };
                },
                error.EndOfStream => {
                    break;
                },
                else => unreachable,
            };

            const line = std.mem.trim(u8, raw_line, spaces);
            if (line.len == 0 or line[0] == '#') {
                // After an empty line, we consider the end of a value.
                need_key = true;
                continue;
            }

            const eq = std.mem.indexOfScalar(u8, line, '=') orelse {
                return .{
                    .err = .{
                        .msg = "invalid variable definition (no '=' found)",
                        .line = file_line,
                    },
                };
            };

            const key = std.mem.trimRight(u8, line[0..eq], spaces);
            const val = std.mem.trimLeft(u8, line[eq + 1 ..], spaces);

            if (key.len == 0) {
                // We have a value continuation here. So we append a newline
                // to the existing value and write the value line below.
                if (need_key) {
                    return .{
                        .err = .{
                            .msg = "missing key name",
                            .line = file_line,
                        },
                    };
                }
                header.val_len += 1;
                try allocating.writer.writeByte('\n');
            } else {
                // We have a new key/value pair starting here. First, we need
                // to write the header from the previous key/value pair if there
                // was some.
                if (allocating.writer.end > 0) {
                    @memcpy(allocating.writer.buffer[header_off .. header_off + @sizeOf(Var_Header)], &header.encode());
                }

                header_off = allocating.writer.end;
                header.key_len = @intCast(key.len);
                header.val_len = 0;

                try allocating.writer.writeAll(&[_]u8{0} ** @sizeOf(Var_Header));
                try allocating.writer.writeAll(key);
                need_key = false;
            }

            header.val_len += @intCast(val.len);
            try allocating.writer.writeAll(val);
        }

        if (allocating.writer.end > 0) {
            @memcpy(allocating.writer.buffer[header_off .. header_off + @sizeOf(Var_Header)], &header.encode());
        }
        return .{
            .conf = .{
                .allocator = allocator,
                .buf = allocating.writer.buffer,
                .len = allocating.writer.end,
            },
        };
    }

    pub fn deinit(self: Config) void {
        if (self.buf.len > 0) {
            self.allocator.free(self.buf);
        }
    }

    pub fn iterate(self: Config) Iterator {
        return .{ .rest = self.buf[0..self.len] };
    }

    pub fn add_var(self: *Config, key: []const u8, val: []const u8) !void {
        const var_size = @sizeOf(Var_Header) + key.len + val.len;
        if (self.len + var_size > self.buf.len) {
            const new_size = @max(256, self.buf.len + @max(var_size, self.buf.len / 2));
            self.buf = try self.allocator.realloc(self.buf, new_size);
        }
        assert(self.len + var_size <= self.buf.len);

        const header: Var_Header = .{
            .key_len = @intCast(key.len),
            .val_len = @intCast(val.len),
        };
        @memcpy(self.buf[self.len .. self.len + 8], &header.encode());
        self.len += 8;
        @memcpy(self.buf[self.len .. self.len + key.len], key);
        self.len += key.len;
        @memcpy(self.buf[self.len .. self.len + val.len], val);
        self.len += val.len;
    }

    /// Find a variable by its key and returns the first variable that
    /// is found. If the key cannot be found, null will be returned.
    pub fn find_var(self: Config, key: []const u8) ?Var {
        var iter = self.iterate();
        while (iter.next()) |v| {
            if (std.mem.eql(u8, v.key, key)) {
                return v;
            }
        }
        return null;
    }
};

test "parse config - empty" {
    var input = std.io.Reader.fixed("");
    var buf: [32]u8 = undefined;
    var reader = std.io.Reader.limited(&input, std.io.Limit.limited(input.buffer.len), &buf);

    var result = try Config.parse(std.testing.allocator, &reader.interface, buf.len);
    try std.testing.expect(result == .conf);
    try std.testing.expectEqualSlices(u8, "", result.conf.buf[0..result.conf.len]);
    defer result.conf.deinit();

    var vars = result.conf.iterate();
    try std.testing.expect(vars.next() == null);
}

test "parse config - singe line" {
    var input = std.io.Reader.fixed("foo one = bar one");
    var buf: [32]u8 = undefined;
    var reader = std.io.Reader.limited(&input, std.io.Limit.limited(input.buffer.len), &buf);

    var result = try Config.parse(std.testing.allocator, &reader.interface, buf.len);
    try std.testing.expect(result == .conf);
    try std.testing.expectEqualSlices(
        u8,
        "\x00\x00\x00\x07\x00\x00\x00\x07foo onebar one",
        result.conf.buf[0..result.conf.len],
    );
    defer result.conf.deinit();

    var vars = result.conf.iterate();

    const var_one = vars.next() orelse return error.missing_var_one;
    try std.testing.expectEqualSlices(u8, "foo one", var_one.key);
    try std.testing.expectEqualSlices(u8, "bar one", var_one.val);

    try std.testing.expectEqual(@as(?Var, null), vars.next());
}

test "parse config - mixed" {
    var input = std.io.Reader.fixed(
        \\foo_one = bar one
        \\foo_two = bar two 1
        \\        = bar two 2
        \\        =
        \\        = bar two 4
        \\
        \\foo_three =
        \\          = bar three 2
        \\          = bar three 3
        \\
        \\
        \\
        \\foo_four = bar four 1
        \\         = bar four 2
        \\         = bar four 3
        \\         =
        \\
        \\foo_five =
        \\
    );
    var buf: [32]u8 = undefined;
    var reader = std.io.Reader.limited(&input, std.io.Limit.limited(input.buffer.len), &buf);

    var result = try Config.parse(std.testing.allocator, &reader.interface, buf.len);
    try std.testing.expect(result == .conf);
    try std.testing.expectEqualSlices(
        u8,
        "" ++
            "\x00\x00\x00\x07\x00\x00\x00\x07foo_onebar one" ++
            "\x00\x00\x00\x07\x00\x00\x00\x1efoo_twobar two 1\nbar two 2\n\nbar two 4" ++
            "\x00\x00\x00\x09\x00\x00\x00\x18foo_three\nbar three 2\nbar three 3" ++
            "\x00\x00\x00\x08\x00\x00\x00\x21foo_fourbar four 1\nbar four 2\nbar four 3\n" ++
            "\x00\x00\x00\x08\x00\x00\x00\x00foo_five",
        result.conf.buf[0..result.conf.len],
    );
    defer result.conf.deinit();

    var vars = result.conf.iterate();

    const var_one = vars.next() orelse return error.missing_var_one;
    try std.testing.expectEqualSlices(u8, "foo_one", var_one.key);
    try std.testing.expectEqualSlices(u8, "bar one", var_one.val);

    const var_two = vars.next() orelse return error.missing_var_two;
    try std.testing.expectEqualSlices(u8, "foo_two", var_two.key);
    try std.testing.expectEqualSlices(u8, "bar two 1\nbar two 2\n\nbar two 4", var_two.val);

    const var_three = vars.next() orelse return error.missing_var_three;
    try std.testing.expectEqualSlices(u8, "foo_three", var_three.key);
    try std.testing.expectEqualSlices(u8, "\nbar three 2\nbar three 3", var_three.val);

    const var_four = vars.next() orelse return error.missing_var_four;
    try std.testing.expectEqualSlices(u8, "foo_four", var_four.key);
    try std.testing.expectEqualSlices(u8, "bar four 1\nbar four 2\nbar four 3\n", var_four.val);

    const var_five = vars.next() orelse return error.missing_var_five;
    try std.testing.expectEqualSlices(u8, "foo_five", var_five.key);
    try std.testing.expectEqualSlices(u8, "", var_five.val);

    try std.testing.expectEqual(@as(?Var, null), vars.next());
}

test "parse config - with comment" {
    var input = std.io.Reader.fixed(
        \\# first var:
        \\foo_one = bar one
        \\# second var:
        \\foo_two = bar two 1
        \\        = bar two 2
        \\        =
        \\        = bar two 4
        \\  # third var:
        \\foo_three =
        \\          = bar three 2
        \\          = bar three 3
    );
    var buf: [32]u8 = undefined;
    var reader = std.io.Reader.limited(&input, std.io.Limit.limited(input.buffer.len), &buf);

    var result = try Config.parse(std.testing.allocator, &reader.interface, buf.len);
    try std.testing.expect(result == .conf);
    try std.testing.expectEqualSlices(
        u8,
        "" ++
            "\x00\x00\x00\x07\x00\x00\x00\x07foo_onebar one" ++
            "\x00\x00\x00\x07\x00\x00\x00\x1efoo_twobar two 1\nbar two 2\n\nbar two 4" ++
            "\x00\x00\x00\x09\x00\x00\x00\x18foo_three\nbar three 2\nbar three 3",
        result.conf.buf[0..result.conf.len],
    );
    defer result.conf.deinit();

    var vars = result.conf.iterate();

    const var_one = vars.next() orelse return error.missing_var_one;
    try std.testing.expectEqualSlices(u8, "foo_one", var_one.key);
    try std.testing.expectEqualSlices(u8, "bar one", var_one.val);

    const var_two = vars.next() orelse return error.missing_var_two;
    try std.testing.expectEqualSlices(u8, "foo_two", var_two.key);
    try std.testing.expectEqualSlices(u8, "bar two 1\nbar two 2\n\nbar two 4", var_two.val);

    const var_three = vars.next() orelse return error.missing_var_three;
    try std.testing.expectEqualSlices(u8, "foo_three", var_three.key);
    try std.testing.expectEqualSlices(u8, "\nbar three 2\nbar three 3", var_three.val);

    try std.testing.expectEqual(@as(?Var, null), vars.next());
}