Joseph Ferano
594a42b54e
(map-remove! m k) answers the value that was there, or None, which is the answer get already gives and for the same reason: a key that is not in the map is an answer, not a failure. Handing the value back rather than dropping it makes "take this out and use it" one call instead of two that hash the key twice. The removal shifts the probe run back over the hole. A Robin Hood lookup stops at the first empty slot, so a hole left in the middle of a run hides every entry after it — and the hidden ones are precisely what a test that only asks after what it removed never looks at, which is why the program removes a thousand of two thousand keys and then asks for the other thousand. Odin was read rather than recalled here, and it does the opposite: its erase marks a tombstone and its insert carries the repair loop. Staying tombstone- free keeps the shape the rest of the file already assumed, and the lookups — which outnumber the removals — pay nothing for it. The note in the runtime and the two in BUILT.md that said Odin deletes by backward shift were describing Odin's insert, and now say which is which. It allocates nothing and releases nothing, so there is no guard around it and it means the same thing on a map in an arena as on one in the heap: a key and a value live inside the one block the map allocated, and there was never anything per entry to hand back.
122 lines
5.5 KiB
Plaintext
122 lines
5.5 KiB
Plaintext
;;;; (map-remove! m k) — the operation a Map has been missing.
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;;;;
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;;;; Removal is the one map operation that can break the *other* ones: Robin
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;;;; Hood lookups stop at the first empty slot, so a hole punched in the middle
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;;;; of a probe run hides every entry after it, and the entries it hides are
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;;;; found by no test that only removes and asks about what it removed. So the
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;;;; rows below are mostly about the survivors.
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;;;;
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;;;; Odin marks a tombstone and repairs on the next insert; this shifts the run
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;;;; back and stays tombstone-free, which is the arrangement the rest of the
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;;;; file already assumed. A version that punched the hole and left it passes
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;;;; row 1 and fails row 3.
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(defstruct Cell [x i32 y i32])
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(defn main [] i32
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;; (1) The value comes back, the length drops, and the key is gone. An
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;; absent key is None and changes nothing — the same answer get gives, since
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;; a key that was not there is an answer and not a failure.
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(let [m (map-new i32 i64)]
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(put m 1 100)
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(put m 2 200)
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(match (map-remove! m 1)
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(Some v) (do (print v) (println "")) ; 100
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None (println "missing"))
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(print (len m)) (println "") ; 1
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(print (has-key? m 1)) (println "") ; false
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(match (map-remove! m 1) (Some v) (do (print v) (println "")) None (println "gone"))
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(println (len m)) ; gone, then 1
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(free m))
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;; (2) Removing every entry empties the map, and it is usable afterwards:
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;; the block is still there and the slots are empty rather than poisoned.
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(let [m (map-new i32 i32)]
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(dotimes [i 64] (put m i i))
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(dotimes [i 64] (map-remove! m i))
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(print (len m)) (println "") ; 0
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(put m 7 77)
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(match (get m 7) (Some v) (do (print v) (println "")) None (println "?")) ; 77
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(print (len m)) (println "") ; 1
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(free m))
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;; (3) The survivors, which is the row that matters. 2000 entries is eight
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;; grows, so the runs are long and interleaved; removing the even keys and
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;; then asking after every odd one is asking whether any probe run was cut.
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;; A backward shift that stopped one slot early loses entries here and
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;; nowhere else.
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(let [m (map-new i32 i64)]
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(dotimes [i 2000] (put m i (* (i64 i) 3)))
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(let [taken 0]
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(dotimes [i 2000]
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(if (= 0 (% i 2))
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(match (map-remove! m i)
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(Some v) (if (= v (* (i64 i) 3)) (set taken (+ taken 1)))
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None (set taken taken))))
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(print taken) (println "")) ; 1000
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(print (len m)) (println "") ; 1000
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(let [lost 0 ghosts 0]
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(dotimes [i 2000]
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(match (get m i)
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(Some v) (if (or (= 0 (% i 2)) (not (= v (* (i64 i) 3))))
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(set ghosts (+ ghosts 1)))
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None (if (not (= 0 (% i 2))) (set lost (+ lost 1)))))
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(print lost) (println "") ; 0
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(print ghosts) (println "")) ; 0
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;; Re-inserting what was taken out puts the length back, through no grow:
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;; the block still has the room the removals freed up.
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(dotimes [i 2000] (if (= 0 (% i 2)) (put m i (* (i64 i) 3))))
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(print (len m)) (println "") ; 2000
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(free m))
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;; (4) A struct key and a string key, so the emitted hash/equality pair is
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;; on the removal path too and not only on get's.
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(let [g (map-new Cell i32)]
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(dotimes [i 20] (dotimes [j 20] (put g (Cell {.x i .y j}) (+ (* i 100) j))))
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(match (map-remove! g (Cell {.x 7 .y 9}))
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(Some v) (do (print v) (println "")) ; 709
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None (println "?"))
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(print (has-key? g (Cell {.x 7 .y 9}))) (println "") ; false
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(print (has-key? g (Cell {.x 7 .y 10}))) (println "") ; true
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(print (len g)) (println "") ; 399
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(free g))
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(let [s (map-new string i32)]
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(put s "alpha" 1)
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(put s "beta" 2)
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(match (map-remove! s "alpha")
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(Some v) (do (print v) (println "")) ; 1
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None (println "?"))
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(print (has-key? s "beta")) (println "") ; true
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(print (len s)) (println "") ; 1
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(free s))
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;; (5) Iteration after removals answers exactly the survivors. The cursor is
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;; started fresh — a cursor held *across* a removal is invalidated by the
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;; shift, the same way a put that grows invalidates one.
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(let [m (map-new i32 i32)]
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(dotimes [i 100] (put m i i))
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(dotimes [i 100] (if (= 0 (% i 3)) (map-remove! m i)))
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(let [cur (i64 0) k 0 v 0 seen 0 sum 0]
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(while (map-next! m (addr cur) (addr k) (addr v))
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(set seen (+ seen 1))
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(if (not (= k v)) (set sum (+ sum 1))))
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(print seen) (println "") ; 66
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(print sum) (println "")) ; 0
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(print (len m)) (println "") ; 66
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(free m))
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;; (6) An arena, which refuses can-free. Removal asks the allocator for
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;; nothing and hands it back nothing — a key and a value live inside the one
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;; block the map allocated — so it means here exactly what it means on the
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;; heap, and the region is released whole as always.
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(let [ar (arena-new 1048576)]
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(with-allocator ar
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(let [t (map-new i32 i32)]
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(dotimes [i 300] (put t i (* i 2)))
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(dotimes [i 300] (if (= 0 (% i 2)) (map-remove! t i)))
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(print (len t)) (println "") ; 150
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(match (get t 299) (Some v) (do (print v) (println "")) None (println "?")) ; 598
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(print (has-key? t 298)) (println ""))) ; false
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(free-all ar))
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0)
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