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Implement HashMap.find_with_or_insert_with() #14196

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merged 5 commits into from
May 16, 2014
Merged

Implement HashMap.find_with_or_insert_with() #14196

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460b552
Reinstate HashMap.mangle().
chris-morgan May 14, 2014
acdce63
Tidy up the HashMap.mangle example.
chris-morgan May 15, 2014
73dc1e0
Rename HashMap.mangle to find_with_or_insert_with.
chris-morgan May 15, 2014
5b13ddb
Use assertions in find_with_or_insert_with example
chris-morgan May 15, 2014
ff98afe
Work around parse error caused by #14240.
chris-morgan May 16, 2014
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79 changes: 58 additions & 21 deletions src/libcollections/hashmap.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1239,31 +1239,14 @@ impl<K: TotalEq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
/// Return the value corresponding to the key in the map, or insert
/// and return the value if it doesn't exist.
pub fn find_or_insert<'a>(&'a mut self, k: K, v: V) -> &'a mut V {
let hash = self.make_hash(&k);
match self.search_hashed(&hash, &k) {
Some(idx) => {
let (_, v_ref) = self.table.read_mut(&idx);
v_ref
},
None => self.insert_hashed(hash, k, v)
}
self.find_with_or_insert_with(k, v, |_k, _v, _a| (), |_k, a| a)
}

/// Return the value corresponding to the key in the map, or create,
/// insert, and return a new value if it doesn't exist.
pub fn find_or_insert_with<'a>(&'a mut self, k: K, f: |&K| -> V)
-> &'a mut V {
let hash = self.make_hash(&k);
match self.search_hashed(&hash, &k) {
Some(idx) => {
let (_, v_ref) = self.table.read_mut(&idx);
v_ref
},
None => {
let v = f(&k);
self.insert_hashed(hash, k, v)
}
}
self.find_with_or_insert_with(k, (), |_k, _v, _a| (), |k, _a| f(k))
}

/// Insert a key-value pair into the map if the key is not already present.
Expand All @@ -1275,12 +1258,66 @@ impl<K: TotalEq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
v: V,
f: |&K, &mut V|)
-> &'a mut V {
self.find_with_or_insert_with(k, v, |k, v, _a| f(k, v), |_k, a| a)
}

/// Modify and return the value corresponding to the key in the map, or
/// insert and return a new value if it doesn't exist.
///
/// This method allows for all insertion behaviours of a hashmap;
/// see methods like `insert`, `find_or_insert` and
/// `insert_or_update_with` for less general and more friendly
/// variations of this.
///
/// # Example
///
/// ```rust
/// use collections::HashMap;
///
/// // map some strings to vectors of strings
/// let mut map = HashMap::new();
/// map.insert("a key", vec!["value"]);
/// map.insert("z key", vec!["value"]);
///
/// let new = vec!["a key", "b key", "z key"];
///
/// for k in new.move_iter() {
/// map.find_with_or_insert_with(
/// k, "new value",
/// // if the key does exist either prepend or append this
/// // new value based on the first letter of the key.
/// |key, already, new| {
/// if key.as_slice().starts_with("z") {
/// already.unshift(new);
/// } else {
/// already.push(new);
/// }
/// },
/// // if the key doesn't exist in the map yet, add it in
/// // the obvious way.
/// |_k, v| vec![v],
/// );
/// }
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This example may be clearer without all the StrBuf, if they're replaced with just &str, does it end up working out?

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I don't see how it would. Prepending/appending the strings would have to produce a StrBuf as a result (as it must allocate), so the value type can't be &str.

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I believe that already has type &mut Vec<StrBuf>, appending/unshifting on a vector, not a string.

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Ah hah, you're right. I didn't read carefully enough.

///
/// for (k, v) in map.iter() {
/// println!("{} -> {}", *k, *v);
/// }
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This would be a little clearer if you asserted equality instead of printing, because I can't see what it prints without actually running the code locally. Instead you could use something like

assert_eq!(map.len(), 3);
assert_eq!(map.get("a key"), vec!["value", "new value"]);
asesrt_eq!(map.get("b key"), vec!["value", "new value"]);
assert_eq!(map.get("z key"), vec!["new value", "value"]);

/// ```
pub fn find_with_or_insert_with<'a, A>(&'a mut self,
k: K,
a: A,
found: |&K, &mut V, A|,
not_found: |&K, A| -> V)
-> &'a mut V {
let hash = self.make_hash(&k);
match self.search_hashed(&hash, &k) {
None => self.insert_hashed(hash, k, v),
None => {
let v = not_found(&k, a);
self.insert_hashed(hash, k, v)
},
Some(idx) => {
let (_, v_ref) = self.table.read_mut(&idx);
f(&k, v_ref);
found(&k, v_ref, a);
v_ref
}
}
Expand Down