tiny.preserves.packed_constants

1//! The tag bytes of the binary syntax, one per kind of value, plus the end marker. Code that writes2//! or checks encoded bytes by hand has to use the same bytes as the packed writer and reader.3const std = @import("std");45/// The tag byte that starts each encoded value, as an enum over `u8`, so code that writes encoded6/// bytes by hand names each tag through it. The packed reader rejects a byte outside these fourteen7/// values as `UnknownTag`.8pub const Tag = enum(u8) {9    false_ = 0x80,10    true_ = 0x81,11    /// The byte 0x84, which closes a record, sequence, set or dictionary. At the start of a value,12    /// the packed reader rejects it as `UnexpectedEndMarker`.13    end = 0x84,14    /// The byte 0x85, which starts an annotation in the binary syntax. The packed writer never15    /// writes it, and the packed reader rejects it as `AnnotationNotSupported`.16    annotation = 0x85,17    /// The byte 0x86, which starts an embedded value. The packed writer writes it before an18    /// embedded value's bytes, which come from its type's `encodePacked`. The packed reader rejects19    /// it as `EmbeddedNotSupported`, so bytes holding an embedded value never decode.20    embedded = 0x86,21    /// The byte 0x87, which starts a double: a length of 8, then the double's eight bytes,22    /// big-endian.23    ieee754 = 0x87,24    signed_integer = 0xb0,25    string = 0xb1,26    byte_string = 0xb2,27    symbol = 0xb3,28    record = 0xb4,29    sequence = 0xb5,30    set = 0xb6,31    dictionary = 0xb7,3233    /// Returns the byte value of the tag, for code that writes a tag into a byte buffer.34    pub fn byte(self: Tag) u8 {35        return @backingInt(self);36    }3738    /// Returns the tag whose byte is `b`, or null otherwise, so the packed reader calls it on each39    /// tag byte for the kind of value that follows. `fromByte(t.byte())` returns `t` for every tag.40    pub fn fromByte(b: u8) ?Tag {41        return switch (b) {42            0x80 => .false_,43            0x81 => .true_,44            0x84 => .end,45            0x85 => .annotation,46            0x86 => .embedded,47            0x87 => .ieee754,48            0xb0 => .signed_integer,49            0xb1 => .string,50            0xb2 => .byte_string,51            0xb3 => .symbol,52            0xb4 => .record,53            0xb5 => .sequence,54            0xb6 => .set,55            0xb7 => .dictionary,56            else => null,57        };58    }59};6061test "Tag round-trips through byte" {62    const all = [_]Tag{63        .false_,         .true_,      .end,         .annotation, .embedded, .ieee754,64        .signed_integer, .string,     .byte_string, .symbol,     .record,   .sequence,65        .set,            .dictionary,66    };67    for (all) |t| {68        try std.testing.expectEqual(t, Tag.fromByte(t.byte()).?);69    }70    try std.testing.expectEqual(@as(?Tag, null), Tag.fromByte(0x00));71    try std.testing.expectEqual(@as(?Tag, null), Tag.fromByte(0x82));72    try std.testing.expectEqual(@as(?Tag, null), Tag.fromByte(0xb8));73}

1pub const packed_constants = @"packed".constants;