1 files changed, 31 insertions, 0 deletions
diff --git a/src/nbc/kahan.sml b/src/nbc/kahan.sml
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+++ b/src/nbc/kahan.sml
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+(* Kahan summation *)
+
+signature KAHAN = sig
+	type t
+	val zero: t
+	val add: t * real -> t
+	val sum: t -> real
+	val sequence: real Sequence.t -> real
+	val list: real list -> real
+	val array: real array -> real
+end
+
+structure Kahan :> KAHAN = struct
+	type t = real * real
+	val zero = (0.0, 0.0)
+	fun add ((s, c), x) =
+		let
+			val y = x - c
+			val t = s + y
+		in
+			(t, t - s - y)
+		end
+	fun sum (s, c) = s
+	local
+		fun swappedAdd (a, b) = add (b, a)
+	in
+		fun sequence e = sum (Sequence.fold swappedAdd zero e)
+		fun list l = sum (foldl swappedAdd zero l)
+		fun array a = sum (Array.foldl swappedAdd zero a)
+	end
+end