Add boolean expression evaluator

Author: CrazyPython

interp.d

@@ -0,0 +1,63 @@
+/+ A boolean expression interpreter.
+ + Used to evaluate boolean bitfields for the query.
+ +/
+module interp;
+
+import std.variant;
+import std.typecons;
+// TCO
+alias BitType = uint;
+enum Op { AND, OR, NOT, ATOM };
+bool eval_form(BitType bitstring, Form form) {
+    if (form.op == Op.AND) {
+        return eval_and(bitstring, form.operands);
+    } else if (form.op == Op.OR) {
+        return eval_or(bitstring, form.operands);
+    } else if (form.op == Op.NOT) {
+        return cast(bool)(!(bitstring & (1 << form.operands[0].bitshift)));
+        //return cast(bool)(!(bitstring & (1 << form.bitshift)));
+    } else {
+        return cast(bool)(bitstring & (1 << form.bitshift));
+    }
+}
+struct Form {
+    Op op;
+    union {
+        Form[] operands;
+        // What is faster, a ubyte or a byte?
+        ubyte bitshift;
+    }
+}
+bool eval_and(BitType bitstring, Form[] terms) {
+    if (terms.length == 0) {
+        return true;
+    }
+    if (eval_form(bitstring, terms[0])) {
+        return eval_and(bitstring, terms[1..$]);
+    } else {
+        // short-circuit evaluation
+        return false;
+    }
+}
+bool eval_or(BitType bitstring, Form[] terms) {
+    if (terms.length == 0) {
+        return false;
+    }
+    if (eval_form(bitstring, terms[0])) {
+        // short-circuit evaluation
+        return true;
+    } else {
+        return eval_or(bitstring, terms[1..$]);
+    }
+}
+private Form Atom(ubyte bitshift) {
+    Form form = { Op.ATOM, bitshift: bitshift };
+    return form;
+}
+unittest {
+    assert(eval_form(0b1, Form(Op.OR, [Atom(0)])) == true);
+    assert(eval_form(0b11, Form(Op.AND, [Atom(0), Atom(1)])) == true);
+    assert(eval_form(0b10, Form(Op.AND, [Atom(0), Atom(1)])) == false);
+    assert(eval_form(0b10, Form(Op.OR , [Atom(0), Atom(1)])) == true);
+    assert(eval_form(0b111,Form(Op.AND, [Form(Op.AND, [Atom(0), Atom(2)]), Atom(1)])) == true);
+}