Joseph Ferano
84e170b349
A sequence library normally returns new sequences. There is no allocator, so every one of these mutates the storage it was handed and a slice is the handle that makes that useful: (slice grid 4 9) is ptr+len into grid, so sorting it sorts those five elements and leaves the rest of grid alone. The test asserts exactly that — it sorts a subslice and prints the whole owning array — because it is the property that would die silently the day a slice parameter started being copied rather than passed by value, and -O2's mem2reg would hide it. Insertion sort rather than anything faster. Quicksort wants a stack and mergesort wants a buffer, and neither exists; insertion sort needs a swap and two indices. It is also the only one of the three whose inner loop is short enough to read, which matters more than the asymptotics on the slice sizes a frame loop actually sorts. The `and` guarding it short-circuits, and that is load-bearing: at j = 0 the left test fails and (at s -1) is never evaluated, so the bounds check never fires. Over [i32] and nothing else. There are no generics, so a second element type is a second copy of all seven functions emitted into every program that links the prelude, and i32 is the type indices, ids and tile values already have. An f32 set waits for a program that wants one. min-i32 and max-i32 return (Option i32) rather than a sentinel because there is no i32 that means "the slice was empty" and is not also a possible element. sum-i32 accumulates in i64 and widens each element explicitly — there is no implicit widening anywhere, and an i32 total over a screenful of i32 is how a sum wraps without anyone noticing.
85 lines
3.5 KiB
Plaintext
85 lines
3.5 KiB
Plaintext
;;;; The prelude's in-place slice algorithms.
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;;;;
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;;;; Every input here is chosen so that a wrong implementation passes nothing.
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;;;; The sort input is unsorted, has duplicates, has negatives and has an odd
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;;;; length, so a comparison with the wrong sense, an off-by-one that drops the
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;;;; last element, and a swap that loses an equal key all show up. The second
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;;;; sort is reverse-sorted, which is the worst case for insertion sort and the
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;;;; case a no-op comparison would pass. The third sorts a *subslice* and then
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;;;; prints the whole owning array: a slice is ptr+len into its owner, so the
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;;;; five elements inside the range must be sorted and the three outside it
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;;;; must be untouched. That last one is the property that dies silently if a
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;;;; slice parameter ever starts being copied.
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(defvar xs [7 i32])
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(defvar ys [5 i32])
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(defvar zs [8 i32])
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(defn show [s [i32]]
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(dotimes [i (len s)]
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(when (> i 0) (print-str " "))
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(print-i64 (i64 (at s i))))
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(newline))
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(defn load-xs []
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(set (at xs 0) 5)
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(set (at xs 1) -3)
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(set (at xs 2) 5)
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(set (at xs 3) 0)
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(set (at xs 4) 12)
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(set (at xs 5) -3)
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(set (at xs 6) 7))
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(defn main [] i32
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(load-xs)
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(show (slice xs 0 (len xs))) ; 5 -3 5 0 12 -3 7
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;; Reading the whole slice, before anything reorders it.
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(print-i64 (sum-i32 (slice xs 0 (len xs)))) (newline) ; 23
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(print-i64 (i64 (match (min-i32 (slice xs 0 (len xs))) (Some v) v None 99)))
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(newline) ; -3
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(print-i64 (i64 (match (max-i32 (slice xs 0 (len xs))) (Some v) v None 99)))
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(newline) ; 12
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;; First index, not the last: 5 appears at 0 and at 2.
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(print-i64 (i64 (match (index-of-i32 (slice xs 0 (len xs)) 5)
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(Some v) v None -1)))
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(newline) ; 0
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(print-i64 (i64 (match (index-of-i32 (slice xs 0 (len xs)) 4)
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(Some v) v None -1)))
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(newline) ; -1
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;; An empty slice has no least element, and None is the answer.
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(print-i64 (i64 (match (min-i32 (slice xs 3 3)) (Some v) v None 99)))
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(newline) ; 99
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;; Reverse of an odd-length slice: the middle element stays put.
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(reverse-i32! (slice xs 0 (len xs)))
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(show (slice xs 0 (len xs))) ; 7 -3 12 0 5 -3 5
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;; And of a two-element one, the smallest case that can actually move.
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(reverse-i32! (slice xs 0 2))
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(show (slice xs 0 (len xs))) ; -3 7 12 0 5 -3 5
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(load-xs)
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(sort-i32! (slice xs 0 (len xs)))
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(show (slice xs 0 (len xs))) ; -3 -3 0 5 5 7 12
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;; Reverse-sorted: the case a comparison that never fires would pass.
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(set (at ys 0) 5) (set (at ys 1) 4) (set (at ys 2) 3)
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(set (at ys 3) 2) (set (at ys 4) 1)
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(sort-i32! (slice ys 0 (len ys)))
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(show (slice ys 0 (len ys))) ; 1 2 3 4 5
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;; A subslice, with the elements on both sides left alone.
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(set (at zs 0) 100) (set (at zs 1) 9) (set (at zs 2) -1)
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(set (at zs 3) 9) (set (at zs 4) 4) (set (at zs 5) 0)
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(set (at zs 6) 200) (set (at zs 7) 300)
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(sort-i32! (slice zs 1 6))
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(show (slice zs 0 (len zs))) ; 100 -1 0 4 9 9 200 300
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;; Degenerate lengths must do nothing rather than run off an end.
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(sort-i32! (slice zs 0 0))
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(reverse-i32! (slice zs 0 0))
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(sort-i32! (slice zs 2 3))
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(reverse-i32! (slice zs 2 3))
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(show (slice zs 0 (len zs))) ; 100 -1 0 4 9 9 200 300
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0)
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