1 files changed, 74 insertions, 0 deletions
diff --git a/src/nbc/history.sml b/src/nbc/history.sml
new file mode 100644
index 0000000..52c1bb2
--- /dev/null
+++ b/src/nbc/history.sml
@@ -0,0 +1,74 @@
+structure History :> sig
+	type history
+	val create: int -> history
+	val clear: history -> history
+	val push: history * char -> string option * history
+end = struct
+	type history = {maximumSize: int, content: string}
+	fun create maximumSize = {maximumSize = maximumSize, content = ""}
+	fun clear {maximumSize, content = _} =
+		{maximumSize = maximumSize, content = ""}
+	fun addToString (string, newCharacter) =
+		let
+			val size = size string
+		in
+			CharVector.tabulate (
+				size + 1
+				, fn index =>
+					if index = size then newCharacter
+					else String.sub (string, index)
+			)
+		end
+	fun shiftIntoString (string, newCharacter) =
+		let
+			val size = size string
+		in
+			CharVector.tabulate (
+				size
+				, fn index =>
+					if index = size - 1 then newCharacter
+					else String.sub (string, index + 1)
+			)
+		end
+	fun push ({maximumSize, content}, newCharacter) =
+		let
+			val currentSize = size content
+		in
+			if currentSize = maximumSize then
+				let
+					val newContent = shiftIntoString (
+						content
+						, newCharacter
+					)
+				in (
+					SOME newContent
+					, {
+						maximumSize = maximumSize
+						, content = newContent
+					}
+				) end
+			else if currentSize = maximumSize - 1 then
+				let
+					val newContent = addToString (
+						content
+						, newCharacter
+					)
+				in (
+					SOME newContent
+					, {
+						maximumSize = maximumSize
+						, content = newContent
+					}
+				) end
+			else (
+				NONE
+				, {
+					maximumSize = maximumSize
+					, content = addToString (
+						content
+						, newCharacter
+					)
+				}
+			)
+		end
+end