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|
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
|
