kahon 0.2.0

A Rust writer for the Kahon binary format.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134

//! Scalar, string, and varuint encoders. All functions append to `buf`.

use crate::error::WriteError;
use crate::types::{
    FALSE, FLOAT32, FLOAT64, INT16, INT32, INT64, INT8, NULL, STRING, TINY_NEG_INT, TINY_STRING,
    TINY_UINT, TRUE, UINT16, UINT32, UINT64, UINT8,
};

// Varuint continuation bit and payload mask
const VARUINT_CONT: u8 = 0x80;
const VARUINT_MASK: u8 = 0x7F;

pub(crate) fn write_null(buf: &mut Vec<u8>) {
    buf.push(NULL);
}

pub(crate) fn write_false(buf: &mut Vec<u8>) {
    buf.push(FALSE);
}

pub(crate) fn write_true(buf: &mut Vec<u8>) {
    buf.push(TRUE);
}

pub(crate) fn write_varuint(buf: &mut Vec<u8>, mut v: u64) {
    while v >= VARUINT_CONT as u64 {
        buf.push(((v as u8) & VARUINT_MASK) | VARUINT_CONT);
        v >>= 7;
    }
    buf.push(v as u8);
}

/// Append `v` as a little-endian unsigned integer occupying `width` bytes.
/// `width` must be 1, 2, 4, or 8, and `v` must fit.
pub(crate) fn write_uintw(buf: &mut Vec<u8>, v: u64, width: usize) {
    let bytes = v.to_le_bytes();
    buf.extend_from_slice(&bytes[..width]);
}

/// Write an integer choosing the smallest tag.
pub(crate) fn write_integer(buf: &mut Vec<u8>, v: i128) -> Result<(), WriteError> {
    // Range rule: the whole-JSON-number range is [-2^63, 2^64 - 1].
    if v < i128::from(i64::MIN) || v > i128::from(u64::MAX) {
        return Err(WriteError::IntegerOutOfRange);
    }

    if (0..=31).contains(&v) {
        buf.push(TINY_UINT + v as u8);
        return Ok(());
    }
    if (-16..=-1).contains(&v) {
        buf.push((i128::from(TINY_NEG_INT) - v) as u8);
        return Ok(());
    }

    if v >= 0 {
        let u = v as u128; // fits since v <= u64::MAX
        if u <= u8::MAX as u128 {
            buf.push(UINT8);
            buf.push(u as u8);
        } else if u <= u16::MAX as u128 {
            buf.push(UINT16);
            buf.extend_from_slice(&(u as u16).to_le_bytes());
        } else if u <= u32::MAX as u128 {
            buf.push(UINT32);
            buf.extend_from_slice(&(u as u32).to_le_bytes());
        } else {
            buf.push(UINT64);
            buf.extend_from_slice(&(u as u64).to_le_bytes());
        }
    } else {
        // v in [i64::MIN, -17]
        let s = v as i64; // fits
        if s >= i8::MIN as i64 {
            buf.push(INT8);
            buf.push(s as i8 as u8);
        } else if s >= i16::MIN as i64 {
            buf.push(INT16);
            buf.extend_from_slice(&(s as i16).to_le_bytes());
        } else if s >= i32::MIN as i64 {
            buf.push(INT32);
            buf.extend_from_slice(&(s as i32).to_le_bytes());
        } else {
            buf.push(INT64);
            buf.extend_from_slice(&s.to_le_bytes());
        }
    }
    Ok(())
}

/// Write a float per ยง5.3. Narrows to f32 if bit-exact round-trip holds,
/// else f64. Rejects NaN/Infinity.
pub(crate) fn write_f64(buf: &mut Vec<u8>, v: f64) -> Result<(), WriteError> {
    if v.is_nan() || v.is_infinite() {
        return Err(WriteError::NaNOrInfinity);
    }
    // Narrowing check: f32 round-trip yielding bit-exact v, including sign of zero.
    let f32v = v as f32;
    let widened = f32v as f64;
    if widened.to_bits() == v.to_bits() {
        buf.push(FLOAT32);
        buf.extend_from_slice(&f32v.to_le_bytes());
    } else {
        buf.push(FLOAT64);
        buf.extend_from_slice(&v.to_le_bytes());
    }
    Ok(())
}

/// TinyString for lengths 1..=15, generic `STRING` otherwise.
pub(crate) fn write_string(buf: &mut Vec<u8>, s: &str) {
    let bytes = s.as_bytes();
    let len = bytes.len();
    if (1..=15).contains(&len) {
        buf.push(TINY_STRING + (len as u8 - 1));
    } else {
        buf.push(STRING);
        write_varuint(buf, len as u64);
    }
    buf.extend_from_slice(bytes);
}

/// Offset width code `w` and the corresponding byte count `W = 2^w`.
pub(crate) fn smallest_width(max_off: u64) -> (u8, usize) {
    if max_off <= u8::MAX as u64 {
        (0, 1)
    } else if max_off <= u16::MAX as u64 {
        (1, 2)
    } else if max_off <= u32::MAX as u64 {
        (2, 4)
    } else {
        (3, 8)
    }
}