1 files changed, 103 insertions, 0 deletions
diff --git a/src/toys/turing/js/tape.js b/src/toys/turing/js/tape.js
new file mode 100644
index 0000000..fd05366
--- /dev/null
+++ b/src/toys/turing/js/tape.js
@@ -0,0 +1,103 @@
+const ESCAPE_REGEX = /[.*+?^${}()|[\]\\]/g;
+
+function escapeForRegex(value) {
+  return value.replace(ESCAPE_REGEX, "\\$&");
+}
+
+export class Tape {
+  constructor({
+    initialContent = "",
+    blankSymbol = "B",
+    minLength = 50,
+    padding = 40
+  } = {}) {
+    this.blankSymbol = blankSymbol;
+    this.minLength = minLength;
+    this.padding = padding;
+    this.reset(initialContent);
+  }
+
+  reset(initialContent = "") {
+    const targetLength = Math.max(this.minLength, initialContent.length + this.padding);
+    this.cells = Array(targetLength).fill(this.blankSymbol);
+    const startOffset = Math.floor((targetLength - initialContent.length) / 2);
+    for (let i = 0; i < initialContent.length; i++) {
+      this.cells[startOffset + i] = initialContent[i];
+    }
+    this.headIndex = startOffset;
+  }
+
+  get length() {
+    return this.cells.length;
+  }
+
+  getHeadIndex() {
+    return this.headIndex;
+  }
+
+  readHead() {
+    return this.getCell(this.headIndex);
+  }
+
+  writeHead(symbol) {
+    this.cells[this.headIndex] = symbol || this.blankSymbol;
+  }
+
+  readAt(index) {
+    return this.getCell(index);
+  }
+
+  writeAt(index, symbol) {
+    if (index < 0) {
+      throw new Error("Cannot write to a negative tape index");
+    }
+    this.ensureRightCapacity(index);
+    this.cells[index] = symbol || this.blankSymbol;
+  }
+
+  setHead(index) {
+    if (index < 0) {
+      throw new Error("Head index cannot be negative");
+    }
+    this.ensureRightCapacity(index);
+    this.headIndex = index;
+  }
+
+  moveLeft() {
+    if (this.headIndex === 0) {
+      this.cells.unshift(this.blankSymbol);
+    } else {
+      this.headIndex -= 1;
+      return;
+    }
+  }
+
+  moveRight() {
+    this.headIndex += 1;
+    if (this.headIndex >= this.cells.length) {
+      this.cells.push(this.blankSymbol);
+    }
+  }
+
+  getCell(index) {
+    if (index < 0 || index >= this.cells.length) {
+      return this.blankSymbol;
+    }
+    return this.cells[index];
+  }
+
+  ensureRightCapacity(index) {
+    while (index >= this.cells.length) {
+      this.cells.push(this.blankSymbol);
+    }
+  }
+
+  getContents({ trimTrailing = true } = {}) {
+    let snapshot = this.cells.join("");
+    if (trimTrailing) {
+      const regex = new RegExp(`${escapeForRegex(this.blankSymbol)}+$`, "g");
+      snapshot = snapshot.replace(regex, "");
+    }
+    return snapshot;
+  }
+}