Share constant_hash code between the meta and codegen crates;

cranelift/codegen/shared/src/constant_hash.rs

@@ -0,0 +1,79 @@
+//! Build support for precomputed constant hash tables.
+//!
+//! This module can generate constant hash tables using open addressing and quadratic probing.
+//!
+//! The hash tables are arrays that are guaranteed to:
+//!
+//! - Have a power-of-two size.
+//! - Contain at least one empty slot.
+//!
+//! This module provides build meta support for lookups in these tables, as well as the shared hash
+//! function used for probing.
+
+use std::iter;
+
+/// A primitive hash function for matching opcodes.
+pub fn simple_hash(s: &str) -> usize {
+    let mut h: u32 = 5381;
+    for c in s.chars() {
+        h = (h ^ c as u32).wrapping_add(h.rotate_right(6));
+    }
+    h as usize
+}
+
+/// Compute an open addressed, quadratically probed hash table containing
+/// `items`. The returned table is a list containing the elements of the
+/// iterable `items` and `None` in unused slots.
+pub fn generate_table<'cont, T, I: iter::Iterator<Item = &'cont T>, H: Fn(&T) -> usize>(
+    items: I,
+    num_items: usize,
+    hash_function: H,
+) -> Vec<Option<&'cont T>> {
+    let size = (1.20 * num_items as f64) as usize;
+    // TODO do we really need the multiply by two here?
+    let size = if size.is_power_of_two() {
+        size * 2
+    } else {
+        size.next_power_of_two()
+    };
+
+    let mut table = vec![None; size];
+
+    for i in items {
+        let mut h = hash_function(&i) % size;
+        let mut s = 0;
+        while table[h].is_some() {
+            s += 1;
+            h = (h + s) % size;
+        }
+        table[h] = Some(i);
+    }
+
+    table
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{generate_table, simple_hash};
+
+    #[test]
+    fn basic() {
+        assert_eq!(simple_hash("Hello"), 0x2fa70c01);
+        assert_eq!(simple_hash("world"), 0x5b0c31d5);
+    }
+
+    #[test]
+    fn test_generate_table() {
+        let v = vec!["Hello".to_string(), "world".to_string()];
+        let table = generate_table(v.iter(), v.len(), |s| simple_hash(&s));
+        assert_eq!(
+            table,
+            vec![
+                None,
+                Some(&"Hello".to_string()),
+                Some(&"world".to_string()),
+                None
+            ]
+        );
+    }
+}