From a11485e51063a4c9bc3fa767763c59fdd22394d7 Mon Sep 17 00:00:00 2001 From: Nick Treleaven Date: Sat, 25 Mar 2023 12:05:31 +0000 Subject: [PATCH] [std.algorithm.searching] Add extrema --- std/algorithm/searching.d | 69 +++++++++++++++++++++++++++++++++++++-- 1 file changed, 67 insertions(+), 2 deletions(-) diff --git a/std/algorithm/searching.d b/std/algorithm/searching.d index 15f7ca99e66..1aceda40d60 100644 --- a/std/algorithm/searching.d +++ b/std/algorithm/searching.d @@ -33,6 +33,7 @@ $(T2 commonPrefix, `commonPrefix("parakeet", "parachute")` returns `"para"`.) $(T2 endsWith, `endsWith("rocks", "ks")` returns `true`.) +$(T2 extrema, `extrema([2, 1, 3, 5, 4])` returns `[1, 5]`.) $(T2 find, `find("hello world", "or")` returns `"orld"` using linear search. (For binary search refer to $(REF SortedRange, std,range).)) @@ -3614,7 +3615,7 @@ Note: See_Also: - $(LREF maxElement), $(REF min, std,algorithm,comparison), $(LREF minCount), + $(LREF extrema), $(LREF maxElement), $(REF min, std,algorithm,comparison), $(LREF minCount), $(LREF minIndex), $(LREF minPos) */ auto minElement(alias map = (a => a), Range)(Range r) @@ -3754,7 +3755,7 @@ Note: See_Also: - $(LREF minElement), $(REF max, std,algorithm,comparison), $(LREF maxCount), + $(LREF extrema), $(LREF minElement), $(REF max, std,algorithm,comparison), $(LREF maxCount), $(LREF maxIndex), $(LREF maxPos) */ auto maxElement(alias map = (a => a), Range)(Range r) @@ -3860,6 +3861,70 @@ if (isInputRange!Range && !isInfinite!Range && assert(arr.maxElement!"a.val".val == 1); } +/** Returns an array of the minimum and maximum element in `r`. + * Makes `< 3n/2` comparisons. + */ +// TODO alias map = a => a +ElementType!Range[2] extrema(Range)(Range r) +if (isRandomAccessRange!Range && hasLength!Range) +in (!r.empty) +{ + if (r.length == 1) + return [r[0], r[0]]; + + typeof(return) result; + size_t i; + if (r.length & 1) // odd + { + result = [r[0], r[0]]; + i = 1; + } + else + { + result = (r[0] < r[1]) ? [r[0], r[1]] : [r[1], r[0]]; + i = 2; + } + // iterate pairs + const imax = r.length; + for (; i != imax; i += 2) + { + // save work + if (r[i] < r[i+1]) + { + if (r[i] < result[0]) + result[0] = r[i]; + if (r[i+1] > result[1]) + result[1] = r[i+1]; + } + else + { + if (r[i+1] < result[0]) + result[0] = r[i+1]; + if (r[i] > result[1]) + result[1] = r[i]; + } + } + return result; +} + +unittest +{ + assert(extrema([5,2,9,4,1]) == [1, 9]); + assert(extrema([8,3,7,4,9]) == [3, 9]); + assert(extrema([1,5,3,2]) == [1, 5]); + assert(extrema([2,3,3,2]) == [2, 3]); + + version (StdRandomTests) + foreach (i; 0..1000) + { + import std.random, std.range; + auto arr = generate!(() => uniform(0, 100)).takeExactly(uniform(1, 10)).array; + auto result = arr.extrema; + assert(result[0] == arr.minElement); + assert(result[1] == arr.maxElement); + } +} + // minPos /** Computes a subrange of `range` starting at the first occurrence of `range`'s