    //! A software version formatted according to the Semantic Version 2 specification.
    //!
    //! See: https://semver.org

    const std = @import("std");
    const Version = @This();

    major: usize,
    minor: usize,
    patch: usize,
    pre: ?[]const u8 = null,
    build: ?[]const u8 = null,

    pub const Range = struct {
        min: Version,
        max: Version,

        pub fn includesVersion(self: Range, ver: Version) bool {
            if (self.min.order(ver) == .gt) return false;
            if (self.max.order(ver) == .lt) return false;
            return true;
        }

        /// Checks if system is guaranteed to be at least `version` or older than `version`.
        /// Returns `null` if a runtime check is required.
        pub fn isAtLeast(self: Range, ver: Version) ?bool {
            if (self.min.order(ver) != .lt) return true;
            if (self.max.order(ver) == .lt) return false;
            return null;
        }
    };

    pub fn order(lhs: Version, rhs: Version) std.math.Order {
        if (lhs.major < rhs.major) return .lt;
        if (lhs.major > rhs.major) return .gt;
        if (lhs.minor < rhs.minor) return .lt;
        if (lhs.minor > rhs.minor) return .gt;
        if (lhs.patch < rhs.patch) return .lt;
        if (lhs.patch > rhs.patch) return .gt;
        if (lhs.pre != null and rhs.pre == null) return .lt;
        if (lhs.pre == null and rhs.pre == null) return .eq;
        if (lhs.pre == null and rhs.pre != null) return .gt;

        // Iterate over pre-release identifiers until a difference is found.

        var lhs_pre_it = std.mem.split(u8, lhs.pre.?, ".");
        var rhs_pre_it = std.mem.split(u8, rhs.pre.?, ".");
        while (true) {
            const next_lid = lhs_pre_it.next();
            const next_rid = rhs_pre_it.next();

            // A larger set of pre-release fields has a higher precedence than a smaller set.

            if (next_lid == null and next_rid != null) return .lt;
            if (next_lid == null and next_rid == null) return .eq;
            if (next_lid != null and next_rid == null) return .gt;

            const lid = next_lid.?; // Left identifier

            const rid = next_rid.?; // Right identifier

            // Attempt to parse identifiers as numbers. Overflows are checked by parse.

            const lnum: ?usize = std.fmt.parseUnsigned(usize, lid, 10) catch |err| switch (err) {
                error.InvalidCharacter => null,
                error.Overflow => unreachable,
            };
            const rnum: ?usize = std.fmt.parseUnsigned(usize, rid, 10) catch |err| switch (err) {
                error.InvalidCharacter => null,
                error.Overflow => unreachable,
            };

            // Numeric identifiers always have lower precedence than non-numeric identifiers.

            if (lnum != null and rnum == null) return .lt;
            if (lnum == null and rnum != null) return .gt;

            // Identifiers consisting of only digits are compared numerically.

            // Identifiers with letters or hyphens are compared lexically in ASCII sort order.

            if (lnum != null and rnum != null) {
                if (lnum.? < rnum.?) return .lt;
                if (lnum.? > rnum.?) return .gt;
            } else {
                const ord = std.mem.order(u8, lid, rid);
                if (ord != .eq) return ord;
            }
        }
    }

    pub fn parse(text: []const u8) !Version {
        // Parse the required major, minor, and patch numbers.

        const extra_index = std.mem.indexOfAny(u8, text, "-+");
        const required = text[0..(extra_index orelse text.len)];
        var it = std.mem.split(u8, required, ".");
        var ver = Version{
            .major = try parseNum(it.first()),
            .minor = try parseNum(it.next() orelse return error.InvalidVersion),
            .patch = try parseNum(it.next() orelse return error.InvalidVersion),
        };
        if (it.next() != null) return error.InvalidVersion;
        if (extra_index == null) return ver;

        // Slice optional pre-release or build metadata components.

        const extra: []const u8 = text[extra_index.?..text.len];
        if (extra[0] == '-') {
            const build_index = std.mem.indexOfScalar(u8, extra, '+');
            ver.pre = extra[1..(build_index orelse extra.len)];
            if (build_index) |idx| ver.build = extra[(idx + 1)..];
        } else {
            ver.build = extra[1..];
        }

        // Check validity of optional pre-release identifiers.

        // See: https://semver.org/#spec-item-9

        if (ver.pre) |pre| {
            it = std.mem.split(u8, pre, ".");
            while (it.next()) |id| {
                // Identifiers MUST NOT be empty.

                if (id.len == 0) return error.InvalidVersion;

                // Identifiers MUST comprise only ASCII alphanumerics and hyphens [0-9A-Za-z-].

                for (id) |c| if (!std.ascii.isAlNum(c) and c != '-') return error.InvalidVersion;

                // Numeric identifiers MUST NOT include leading zeroes.

                const is_num = for (id) |c| {
                    if (!std.ascii.isDigit(c)) break false;
                } else true;
                if (is_num) _ = try parseNum(id);
            }
        }

        // Check validity of optional build metadata identifiers.

        // See: https://semver.org/#spec-item-10

        if (ver.build) |build| {
            it = std.mem.split(u8, build, ".");
            while (it.next()) |id| {
                // Identifiers MUST NOT be empty.

                if (id.len == 0) return error.InvalidVersion;

                // Identifiers MUST comprise only ASCII alphanumerics and hyphens [0-9A-Za-z-].

                for (id) |c| if (!std.ascii.isAlNum(c) and c != '-') return error.InvalidVersion;
            }
        }

        return ver;
    }

    fn parseNum(text: []const u8) !usize {
        // Leading zeroes are not allowed.

        if (text.len > 1 and text[0] == '0') return error.InvalidVersion;

        return std.fmt.parseUnsigned(usize, text, 10) catch |err| switch (err) {
            error.InvalidCharacter => return error.InvalidVersion,
            else => |e| return e,
        };
    }

    pub fn format(
        self: Version,
        comptime fmt: []const u8,
        options: std.fmt.FormatOptions,
        out_stream: anytype,
    ) !void {
        _ = options;
        if (fmt.len != 0) @compileError("Unknown format string: '" ++ fmt ++ "'");
        try std.fmt.format(out_stream, "{d}.{d}.{d}", .{ self.major, self.minor, self.patch });
        if (self.pre) |pre| try std.fmt.format(out_stream, "-{s}", .{pre});
        if (self.build) |build| try std.fmt.format(out_stream, "+{s}", .{build});
    }

    const expect = std.testing.expect;
    const expectError = std.testing.expectError;

    test "SemanticVersion format" {
        // Test vectors are from https://github.com/semver/semver.org/issues/59#issuecomment-390854010.

        // Valid version strings should be accepted.

        for ([_][]const u8{
            "0.0.4",
            "1.2.3",
            "10.20.30",
            "1.1.2-prerelease+meta",
            "1.1.2+meta",
            "1.1.2+meta-valid",
            "1.0.0-alpha",
            "1.0.0-beta",
            "1.0.0-alpha.beta",
            "1.0.0-alpha.beta.1",
            "1.0.0-alpha.1",
            "1.0.0-alpha0.valid",
            "1.0.0-alpha.0valid",
            "1.0.0-alpha-a.b-c-somethinglong+build.1-aef.1-its-okay",
            "1.0.0-rc.1+build.1",
            "2.0.0-rc.1+build.123",
            "1.2.3-beta",
            "10.2.3-DEV-SNAPSHOT",
            "1.2.3-SNAPSHOT-123",
            "1.0.0",
            "2.0.0",
            "1.1.7",
            "2.0.0+build.1848",
            "2.0.1-alpha.1227",
            "1.0.0-alpha+beta",
            "1.2.3----RC-SNAPSHOT.12.9.1--.12+788",
            "1.2.3----R-S.12.9.1--.12+meta",
            "1.2.3----RC-SNAPSHOT.12.9.1--.12",
            "1.0.0+0.build.1-rc.10000aaa-kk-0.1",
        }) |valid| try std.testing.expectFmt(valid, "{}", .{try parse(valid)});

        // Invalid version strings should be rejected.

        for ([_][]const u8{
            "",
            "1",
            "1.2",
            "1.2.3-0123",
            "1.2.3-0123.0123",
            "1.1.2+.123",
            "+invalid",
            "-invalid",
            "-invalid+invalid",
            "-invalid.01",
            "alpha",
            "alpha.beta",
            "alpha.beta.1",
            "alpha.1",
            "alpha+beta",
            "alpha_beta",
            "alpha.",
            "alpha..",
            "beta\\",
            "1.0.0-alpha_beta",
            "-alpha.",
            "1.0.0-alpha..",
            "1.0.0-alpha..1",
            "1.0.0-alpha...1",
            "1.0.0-alpha....1",
            "1.0.0-alpha.....1",
            "1.0.0-alpha......1",
            "1.0.0-alpha.......1",
            "01.1.1",
            "1.01.1",
            "1.1.01",
            "1.2",
            "1.2.3.DEV",
            "1.2-SNAPSHOT",
            "1.2.31.2.3----RC-SNAPSHOT.12.09.1--..12+788",
            "1.2-RC-SNAPSHOT",
            "-1.0.3-gamma+b7718",
            "+justmeta",
            "9.8.7+meta+meta",
            "9.8.7-whatever+meta+meta",
        }) |invalid| try expectError(error.InvalidVersion, parse(invalid));

        // Valid version string that may overflow.

        const big_valid = "99999999999999999999999.999999999999999999.99999999999999999";
        if (parse(big_valid)) |ver| {
            try std.testing.expectFmt(big_valid, "{}", .{ver});
        } else |err| try expect(err == error.Overflow);

        // Invalid version string that may overflow.

        const big_invalid = "99999999999999999999999.999999999999999999.99999999999999999----RC-SNAPSHOT.12.09.1--------------------------------..12";
        if (parse(big_invalid)) |ver| std.debug.panic("expected error, found {}", .{ver}) else |_| {}
    }

    test "SemanticVersion precedence" {
        // SemVer 2 spec 11.2 example: 1.0.0 < 2.0.0 < 2.1.0 < 2.1.1.

        try expect(order(try parse("1.0.0"), try parse("2.0.0")) == .lt);
        try expect(order(try parse("2.0.0"), try parse("2.1.0")) == .lt);
        try expect(order(try parse("2.1.0"), try parse("2.1.1")) == .lt);

        // SemVer 2 spec 11.3 example: 1.0.0-alpha < 1.0.0.

        try expect(order(try parse("1.0.0-alpha"), try parse("1.0.0")) == .lt);

        // SemVer 2 spec 11.4 example: 1.0.0-alpha < 1.0.0-alpha.1 < 1.0.0-alpha.beta < 1.0.0-beta <

        // 1.0.0-beta.2 < 1.0.0-beta.11 < 1.0.0-rc.1 < 1.0.0.

        try expect(order(try parse("1.0.0-alpha"), try parse("1.0.0-alpha.1")) == .lt);
        try expect(order(try parse("1.0.0-alpha.1"), try parse("1.0.0-alpha.beta")) == .lt);
        try expect(order(try parse("1.0.0-alpha.beta"), try parse("1.0.0-beta")) == .lt);
        try expect(order(try parse("1.0.0-beta"), try parse("1.0.0-beta.2")) == .lt);
        try expect(order(try parse("1.0.0-beta.2"), try parse("1.0.0-beta.11")) == .lt);
        try expect(order(try parse("1.0.0-beta.11"), try parse("1.0.0-rc.1")) == .lt);
        try expect(order(try parse("1.0.0-rc.1"), try parse("1.0.0")) == .lt);
    }

    test "zig_version" {
        // An approximate Zig build that predates this test.

        const older_version = .{ .major = 0, .minor = 8, .patch = 0, .pre = "dev.874" };

        // Simulated compatibility check using Zig version.

        const compatible = comptime @import("builtin").zig_version.order(older_version) == .gt;
        if (!compatible) @compileError("zig_version test failed");
    }