Add history to pengueno

2 files changed, 37 insertions, 0 deletions

diff --git a/u/history.ts b/u/history.ts
new file mode 100644
index 0000000..5b13961
--- /dev/null
+++ b/u/history.ts
@@ -0,0 +1,36 @@
+export interface History<T> {
+    undo: () => History<T> | undefined;
+    redo: () => History<T> | undefined;
+
+    get: () => T;
+    add: (value: T) => History<T>;
+}
+
+export class HistoryImpl<T> implements History<T> {
+    private readonly item: T;
+    private previous?: History<T>;
+    private next?: History<T>;
+
+    constructor(item: T) {
+        this.item = item;
+    }
+
+    public get(): T {
+        return this.item;
+    }
+
+    public undo(): History<T> | undefined {
+        return this.previous;
+    }
+
+    public redo(): History<T> | undefined {
+        return this.next;
+    }
+
+    public add(value: T): History<T> {
+        const newHistory = new HistoryImpl(value);
+        newHistory.previous = this;
+        this.next = newHistory;
+        return newHistory;
+    }
+}
diff --git a/u/mod.ts b/u/mod.ts
index 8397ce6..2ab8f68 100644
--- a/u/mod.ts
+++ b/u/mod.ts
@@ -3,3 +3,4 @@ export * from "./leftpadesque/mod.ts";
 export * from "./process/mod.ts";
 export * from "./trace/mod.ts";
 export * from "./server/mod.ts";
+export * from "./history.ts";