add Unpack for unpacking integers.

src/implementation/c/code/index.ts

@@ -8,6 +8,7 @@ import { MulAdd } from './mul-add';
 import { Or } from './or';
 import { Store } from './store';
 import { Test } from './test';
+import { Unpack } from './unpack';
 import { Update } from './update';
 
 export * from './base';

src/implementation/c/code/unpack.ts

@@ -0,0 +1,43 @@
+import * as frontend from 'llparse-frontend';
+import * as assert from 'assert';
+import { UNSIGNED_LIMITS, UNSIGNED_TYPES } from '../constants';
+import { Compilation } from '../compilation';
+import { Field } from './field';
+
+export class Unpack extends Field<frontend.code.Unpack>{
+    // big endian will be rshift and little will be lshift
+    protected doBuild(ctx: Compilation, out: string[]): void {
+        const ty = ctx.getFieldType(this.ref.field);
+        assert(UNSIGNED_TYPES.has(ty), `Unexpected Unpack type "${ty}"`);
+        const targetTy = UNSIGNED_TYPES.get(ty)!;
+        const limit = UNSIGNED_LIMITS.get(ty)!;
+        const match = ctx.matchVar();
+
+        /* NOTE: (Vizonex) Feel free to correct me if I'm wrong here about this C code. */
+        if (!this.ref.bigEndian){    
+            /* lshift */
+            out.push(`if (${this.field(ctx)} > 0){`);
+            out.push(`  ${targetTy} __lshift_val = ${this.field(ctx)} << 8;`);
+            out.push(`  if (__lshift_val > ${limit[1]} || __lshift_val < ${this.field(ctx)}){`);
+            out.push("    /* overflow */");
+            out.push("    return 1;");
+            out.push("  }");
+            out.push(`  ${this.field(ctx)} = __lshift_val | ${match}`);
+        } else {
+            out.push(`if (${this.field(ctx)} > 0){`);
+            out.push(`  ${targetTy} __rshift_val = ${this.field(ctx)} >> 8;`);
+            out.push(`  if (__rshift_val > ${limit[1]} || __rshift_val < ${this.field(ctx)}){`);
+            out.push("    /* overflow */");
+            out.push(`    return 1;`);
+            out.push(`  }`);
+            out.push(`  ${this.field(ctx)} =  ${match} | __rshift_val`);
+            out.push(`}`);
+
+        }
+        /** if first value simply setting it in should be enough... */
+        out.push("} else {");
+        out.push(`  ${this.field(ctx)} = ${match};`);
+        out.push("}  ");
+        out.push("return 0;");
+    }
+}