srcmap_codec/
decode.rs

1use crate::vlq::vlq_decode;
2use crate::{DecodeError, Line, Segment, SourceMapMappings};
3
4/// Decode a VLQ-encoded source map mappings string into structured data.
5///
6/// The mappings string uses `;` to separate lines and `,` to separate
7/// segments within a line. Each segment is a sequence of VLQ-encoded
8/// values representing column offsets, source indices, and name indices.
9///
10/// Values are relative (delta-encoded) within the mappings string:
11/// - Generated column resets to 0 for each new line
12/// - Source index, original line, original column, and name index
13///   are cumulative across the entire mappings string
14///
15/// # Errors
16///
17/// Returns [`DecodeError`] if the input contains invalid base64 characters,
18/// truncated VLQ sequences, or values that overflow i64.
19pub fn decode(input: &str) -> Result<SourceMapMappings, DecodeError> {
20    if input.is_empty() {
21        return Ok(Vec::new());
22    }
23
24    let bytes = input.as_bytes();
25    let len = bytes.len();
26
27    // Pre-count lines and segments in a single pass for capacity hints
28    let mut semicolons = 0usize;
29    let mut commas = 0usize;
30    for &b in bytes {
31        semicolons += (b == b';') as usize;
32        commas += (b == b',') as usize;
33    }
34    let line_count = semicolons + 1;
35    let approx_segments = commas + line_count;
36    let avg_segments_per_line = approx_segments / line_count;
37    let mut mappings: SourceMapMappings = Vec::with_capacity(line_count);
38
39    // Cumulative state across the entire mappings string
40    let mut source_index: i64 = 0;
41    let mut original_line: i64 = 0;
42    let mut original_column: i64 = 0;
43    let mut name_index: i64 = 0;
44
45    let mut pos: usize = 0;
46
47    loop {
48        // Generated column resets per line
49        let mut generated_column: i64 = 0;
50        let mut line: Line = Vec::with_capacity(avg_segments_per_line);
51        let mut saw_semicolon = false;
52
53        while pos < len {
54            let byte = bytes[pos];
55
56            if byte == b';' {
57                pos += 1;
58                saw_semicolon = true;
59                break;
60            }
61
62            if byte == b',' {
63                pos += 1;
64                continue;
65            }
66
67            // Field 1: generated column (always present)
68            let (delta, consumed) = vlq_decode(bytes, pos)?;
69            generated_column += delta;
70            pos += consumed;
71
72            // Build segment with exact allocation (1, 4, or 5 fields)
73            let segment: Segment = if pos < len && bytes[pos] != b',' && bytes[pos] != b';' {
74                // Fields 2-4: source, original line, original column
75                let (delta, consumed) = vlq_decode(bytes, pos)?;
76                source_index += delta;
77                pos += consumed;
78
79                let (delta, consumed) = vlq_decode(bytes, pos)?;
80                original_line += delta;
81                pos += consumed;
82
83                let (delta, consumed) = vlq_decode(bytes, pos)?;
84                original_column += delta;
85                pos += consumed;
86
87                // Field 5: name index (optional)
88                if pos < len && bytes[pos] != b',' && bytes[pos] != b';' {
89                    let (delta, consumed) = vlq_decode(bytes, pos)?;
90                    name_index += delta;
91                    pos += consumed;
92                    Segment::five(
93                        generated_column,
94                        source_index,
95                        original_line,
96                        original_column,
97                        name_index,
98                    )
99                } else {
100                    Segment::four(
101                        generated_column,
102                        source_index,
103                        original_line,
104                        original_column,
105                    )
106                }
107            } else {
108                Segment::one(generated_column)
109            };
110
111            line.push(segment);
112        }
113
114        mappings.push(line);
115
116        if !saw_semicolon {
117            break;
118        }
119    }
120
121    Ok(mappings)
122}
srcmap_codec/decode.rs

srcmap_codec/
decode.rs
