A-M baby

Task/Four-bit-adder/JavaScript/four-bit-adder-2.js

@@ -0,0 +1,72 @@
+// basic building blocks allowed by the rules are ~, &, and |, we'll fake these
+// in a way that makes what they do (at least when you use them) more obvious
+// than the other available options do.
+
+function not(a) {
+    if (arePseudoBin(a))
+        return a == 1 ? 0 : 1;
+}
+
+function and(a, b) {
+    if (arePseudoBin(a, b))
+        return a + b < 2 ? 0 : 1;
+}
+
+function nand(a, b) {
+    if (arePseudoBin(a, b))
+        return not(and(a, b));
+}
+
+function or(a, b) {
+    if (arePseudoBin(a, b))
+        return nand(nand(a,a), nand(b,b));
+}
+
+function xor(a, b) {
+    if (arePseudoBin(a, b))
+        return nand(nand(nand(a,b), a), nand(nand(a,b), b));
+}
+
+function halfAdder(a, b) {
+    if (arePseudoBin(a, b))
+        return { carry: and(a, b), sum: xor(a, b) };
+}
+
+function fullAdder(a, b, c) {
+    if (arePseudoBin(a, b, c)) {
+        var h0 = halfAdder(a, b),
+            h1 = halfAdder(h0.sum, c);
+        return {carry: or(h0.carry, h1.carry), sum: h1.sum };
+    }
+}
+
+function fourBitAdder(a, b) {
+    if (typeof a.length == 'undefined' || typeof b.length == 'undefined')
+        throw new Error('bad values');
+    // not sure if the rules allow this, but we need to pad the values
+    // if they're too short and trim them if they're too long
+    var inA = Array(4),
+        inB = Array(4),
+        out = Array(4),
+        i = 4,
+        pass;
+
+    while (i--) {
+        inA[i] = a[i] != 1 ? 0 : 1;
+        inB[i] = b[i] != 1 ? 0 : 1;
+    }
+
+    // now we can start adding... I'd prefer to do this in a loop,
+    // but that wouldn't be "connecting the other 'constructive blocks',
+    // in turn made of 'simpler' and 'smaller' ones"
+
+    pass = halfAdder(inA[3], inB[3]);
+    out[3] = pass.sum;
+    pass = fullAdder(inA[2], inB[2], pass.carry);
+    out[2] = pass.sum;
+    pass = fullAdder(inA[1], inB[1], pass.carry);
+    out[1] = pass.sum;
+    pass = fullAdder(inA[0], inB[0], pass.carry);
+    out[0] = pass.sum;
+    return out.join('');
+}