schema_core/config/
content_hash.rs

1use std::hash::{Hash, Hasher};
2
3/// FNV-1a offset basis.
4const FNV_OFFSET: u32 = 2_166_136_261;
5/// FNV-1a prime.
6const FNV_PRIME: u32 = 16_777_619;
7
8#[derive(Debug, Clone, Copy, Hash, Eq, PartialEq)]
9pub struct ContentHash(u32);
10
11impl ContentHash {
12    pub fn new(value: u32) -> Self {
13        Self(value)
14    }
15
16    /// The content hash of any [`Hash`] value, via FNV-1a. Deterministic for a
17    /// given structure — the same parsed value always hashes the same — which
18    /// is what makes it usable as a stable, structure-derived identifier.
19    pub fn of<T: Hash>(value: &T) -> Self {
20        let mut hasher = Fnv1aHasher::default();
21        value.hash(&mut hasher);
22        Self(hasher.0)
23    }
24}
25
26impl std::fmt::Display for ContentHash {
27    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
28        write!(f, "{:08x}", self.0)
29    }
30}
31
32/// Serializes as the eight-hex-digit string (its [`Display`](std::fmt::Display)) —
33/// the same form that suffixes a physical index name — rather than the raw `u32`.
34impl serde::Serialize for ContentHash {
35    fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
36        serializer.collect_str(self)
37    }
38}
39
40/// An FNV-1a [`Hasher`], so any [`Hash`] value yields a stable [`ContentHash`]
41/// independent of the platform's default (randomized) hasher.
42struct Fnv1aHasher(u32);
43
44impl Default for Fnv1aHasher {
45    fn default() -> Self {
46        Self(FNV_OFFSET)
47    }
48}
49
50impl Hasher for Fnv1aHasher {
51    fn finish(&self) -> u64 {
52        u64::from(self.0)
53    }
54
55    fn write(&mut self, bytes: &[u8]) {
56        for byte in bytes {
57            self.0 ^= u32::from(*byte);
58            self.0 = self.0.wrapping_mul(FNV_PRIME);
59        }
60    }
61}
62
63#[cfg(test)]
64#[allow(clippy::unwrap_used)]
65mod tests {
66    use super::*;
67
68    #[test]
69    fn hash_is_deterministic_for_equal_values() {
70        let a = ContentHash::of(&("users", 1u8, vec!["id", "email"]));
71        let b = ContentHash::of(&("users", 1u8, vec!["id", "email"]));
72        assert_eq!(a, b);
73    }
74
75    #[test]
76    fn hash_changes_when_structure_changes() {
77        let before = ContentHash::of(&vec!["id", "email"]);
78        let after = ContentHash::of(&vec!["id", "email", "name"]);
79        assert_ne!(before, after);
80    }
81
82    #[test]
83    fn display_is_eight_hex_digits() {
84        assert_eq!(format!("{}", ContentHash::new(0xABCD)), "0000abcd");
85    }
86}
schema_core/config/content_hash.rs

schema_core/config/
content_hash.rs
