tiny.css.selector.nth

1const std = @import("std");23/// Which sibling sequence a structural pseudo-class counts over.4pub const Kind = enum(u8) {5    child,6    last_child,7    of_type,8    last_of_type,9};1011/// One `An+B` test. `next` chains further structural tests on the same12/// compound, one based, so that `Compound.nth` stays a single word.13pub const Nth = struct {14    a: i32,15    b: i32,16    kind: Kind,17    next: u32 = 0,18};1920/// The coefficients of an `An+B` expression.21pub const Coefficients = struct {22    a: i32,23    b: i32,24};2526/// Parses `odd`, `even`, `An+B`, `n`, or a bare integer.27pub fn parse(text: []const u8) ?Coefficients {28    const trimmed = std.mem.trim(u8, text, " \t\r\n\x0C");29    if (trimmed.len == 0) return null;30    if (std.ascii.eqlIgnoreCase(trimmed, "odd")) return .{ .a = 2, .b = 1 };31    if (std.ascii.eqlIgnoreCase(trimmed, "even")) return .{ .a = 2, .b = 0 };32    const split = std.mem.indexOfAny(u8, trimmed, "nN") orelse {33        return .{ .a = 0, .b = integer(trimmed) orelse return null };34    };35    const head = std.mem.trim(u8, trimmed[0..split], " \t\r\n\x0C");36    const tail = std.mem.trim(u8, trimmed[split + 1 ..], " \t\r\n\x0C");37    const a = coefficient(head) orelse return null;38    if (tail.len == 0) return .{ .a = a, .b = 0 };39    const sign: i32 = switch (tail[0]) {40        '+' => 1,41        '-' => -1,42        else => return null,43    };44    const rest = std.mem.trim(u8, tail[1..], " \t\r\n\x0C");45    const magnitude = integer(rest) orelse return null;46    if (magnitude < 0) return null;47    return .{ .a = a, .b = sign * magnitude };48}4950/// Whether a one based `index` out of `count` siblings satisfies `An+B`.51pub fn matches(coefficients: Coefficients, index: u32, count: u32, kind: Kind) bool {52    std.debug.assert(index >= 1);53    if (count != 0) std.debug.assert(index <= count);54    const last = kind == .last_child or kind == .last_of_type;55    const position: i64 = if (last)56        @as(i64, count) - @as(i64, index) + 157    else58        @as(i64, index);59    const b: i64 = coefficients.b;60    if (coefficients.a == 0) return position == b;61    const offset = position - b;62    const a: i64 = coefficients.a;63    if (@rem(offset, a) != 0) return false;64    return @divTrunc(offset, a) >= 0;65}6667fn coefficient(head: []const u8) ?i32 {68    if (head.len == 0) return 1;69    if (head.len == 1 and head[0] == '+') return 1;70    if (head.len == 1 and head[0] == '-') return -1;71    return integer(head);72}7374fn integer(text: []const u8) ?i32 {75    if (text.len == 0) return null;76    var index: usize = 0;77    var sign: i32 = 1;78    if (text[0] == '+' or text[0] == '-') {79        sign = if (text[0] == '-') -1 else 1;80        index = 1;81    }82    if (index >= text.len) return null;83    var magnitude: i32 = 0;84    while (index < text.len) : (index += 1) {85        if (!std.ascii.isDigit(text[index])) return null;86        magnitude = std.math.mul(i32, magnitude, 10) catch return null;87        magnitude = std.math.add(i32, magnitude, text[index] - '0') catch return null;88    }89    return sign * magnitude;90}9192test "an nth expression parses its keywords, coefficients, and offsets" {93    try std.testing.expectEqual(Coefficients{ .a = 2, .b = 1 }, parse("odd").?);94    try std.testing.expectEqual(Coefficients{ .a = 2, .b = 0 }, parse("EVEN").?);95    try std.testing.expectEqual(Coefficients{ .a = 0, .b = 3 }, parse("3").?);96    try std.testing.expectEqual(Coefficients{ .a = 3, .b = 1 }, parse("3n + 1").?);97    try std.testing.expectEqual(Coefficients{ .a = -1, .b = 3 }, parse("-n+3").?);98    try std.testing.expectEqual(Coefficients{ .a = 1, .b = 0 }, parse("n").?);99    try std.testing.expectEqual(Coefficients{ .a = 1, .b = -2 }, parse("+n-2").?);100    try std.testing.expect(parse("") == null);101    try std.testing.expect(parse("2x+1") == null);102    try std.testing.expect(parse("2n*1") == null);103}104105test "a fixed index matches only its own position" {106    const first = Coefficients{ .a = 0, .b = 1 };107    try std.testing.expect(matches(first, 1, 5, .child));108    try std.testing.expect(!matches(first, 2, 5, .child));109    try std.testing.expect(matches(first, 5, 5, .last_child));110    try std.testing.expect(!matches(first, 4, 5, .last_child));111}112113test "a stepped expression matches its arithmetic sequence forward only" {114    const odd = Coefficients{ .a = 2, .b = 1 };115    try std.testing.expect(matches(odd, 1, 6, .child));116    try std.testing.expect(!matches(odd, 2, 6, .child));117    try std.testing.expect(matches(odd, 5, 6, .child));118    const leading = Coefficients{ .a = -1, .b = 3 };119    try std.testing.expect(matches(leading, 1, 9, .child));120    try std.testing.expect(matches(leading, 3, 9, .child));121    try std.testing.expect(!matches(leading, 4, 9, .child));122}

1pub const nth = nth_expression;