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feat: real transients backed by Janet arrays/tables (interop)

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Replace the correctness-only transient aliases with real mutable scratch
collections via host interop:
- transient vector -> a Janet array; conj!/assoc!/pop! mutate in place
- transient map -> a Janet table keyed by canonical key (collection keys still
  compare by value); assoc!/dissoc!/conj! mutate in place
- transient set -> a Janet table; conj!/disj! mutate in place
- persistent! freezes back to a pvec / phm / phs
- count/nth/get/contains? work on transients; transient? predicate added

Building a map/set this way avoids the persistent path's per-step bucket-array
copying (transient map build ~35% faster at 20k here); vectors are comparable
since pvec conj is already ~O(1). The mutating ops return the transient and the
source collection is untouched.

spec/transients-spec (34 cases). conformance 218/218, jpm test green.
This commit is contained in:
Yogthos 2026-06-05 08:02:21 -04:00
parent a0943cb944
commit f38d402445
4 changed files with 140 additions and 13 deletions
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# Specification: transients (mutable scratch collections frozen by persistent!).
(use ../support/harness)
(defspec "transient / vector"
["conj! then persistent!" "[1 2]" "(persistent! (conj! (conj! (transient []) 1) 2))"]
["reduce conj!" "[0 1 2 3 4]" "(persistent! (reduce conj! (transient []) (range 5)))"]
["conj! many args" "[1 2 3]" "(persistent! (conj! (transient [1]) 2 3))"]
["assoc! existing" "[1 9 3]" "(persistent! (assoc! (transient [1 2 3]) 1 9))"]
["assoc! at count grows" "[1 2 3]" "(persistent! (assoc! (transient [1 2]) 2 3))"]
["pop!" "[1 2]" "(persistent! (pop! (transient [1 2 3])))"]
["from existing vector" "[1 2 3 4]" "(persistent! (conj! (transient [1 2 3]) 4))"]
["count" "3" "(count (transient [1 2 3]))"]
["nth" "2" "(nth (transient [1 2 3]) 1)"]
["get" "2" "(get (transient [1 2 3]) 1)"]
["persistent! is a vector" "true" "(vector? (persistent! (transient [1])))"]
["transient? true" "true" "(transient? (transient []))"]
["transient? false" "false" "(transient? [1 2])"])
(defspec "transient / map"
["assoc! then persistent!" "{:a 1, :b 2}" "(persistent! (assoc! (assoc! (transient {}) :a 1) :b 2))"]
["assoc! many" "{:a 1, :b 2}" "(persistent! (assoc! (transient {}) :a 1 :b 2))"]
["dissoc!" "{:b 2}" "(persistent! (dissoc! (transient {:a 1 :b 2}) :a))"]
["conj! map entry" "{:a 1}" "(persistent! (conj! (transient {}) [:a 1]))"]
["from existing map" "{:a 1, :b 2}" "(persistent! (assoc! (transient {:a 1}) :b 2))"]
["get" "1" "(get (transient {:a 1}) :a)"]
["get missing default" ":x" "(get (transient {:a 1}) :z :x)"]
["contains?" "true" "(contains? (transient {:a 1}) :a)"]
["count" "2" "(count (transient {:a 1 :b 2}))"]
["collection key by value" ":v" "(get (persistent! (assoc! (transient {}) [1 2] :v)) [1 2])"]
["persistent! is a map" "true" "(map? (persistent! (transient {:a 1})))"]
["reduce build" "{0 0, 1 1, 2 2}" "(persistent! (reduce (fn [t i] (assoc! t i i)) (transient {}) (range 3)))"])
(defspec "transient / set"
["conj! dedups" "#{1 2 3}" "(persistent! (conj! (transient #{}) 1 2 2 3))"]
["disj!" "#{1 3}" "(persistent! (disj! (transient #{1 2 3}) 2))"]
["from existing set" "#{1 2 3}" "(persistent! (conj! (transient #{1 2}) 3))"]
["contains?" "true" "(contains? (transient #{1 2}) 1)"]
["count" "2" "(count (transient #{1 2}))"]
["persistent! is a set" "true" "(set? (persistent! (transient #{1})))"]
["map elements by value" "1" "(count (persistent! (conj! (transient #{}) {:a 1} (hash-map :a 1))))"])
(defspec "transient / immutability of source"
["source vector unchanged" "true"
"(let [v [1 2 3] _ (persistent! (conj! (transient v) 4))] (= v [1 2 3]))"]
["source map unchanged" "true"
"(let [m {:a 1} _ (persistent! (assoc! (transient m) :b 2))] (= m {:a 1}))"])