btc: Add lookup() script utility and some simple composite opcodes

Author: shesek

src/stdlib/btc.minsc

@@ -58,6 +58,36 @@ fn tx::sighash::tr_leaf($tx, $vin, $script_leaf, $utxos, $sighash_ty) = psbt::si
 
 // TOOD tx::sighash::sw0() with just the signed utxo & tx::sighash::presw() with just the utxo's script
 
+//
+// Script utilities
+//
+
+// Verify that the top stack element is exactly 1 or 0
+OP::MINIMAL_BOOL = `OP_DUP OP_SIZE OP_EQUALVERIFY`;
+
+// MUL for small constant numbers
+OP::2MUL = `OP_DUP OP_ADD`;
+OP::3MUL = `OP_DUP*2 OP_ADD*2`;
+OP::4MUL = OP::2MUL*2;
+OP::5MUL = `OP_DUP OP::4MUL OP_ADD`;
+OP::6MUL = `OP::3MUL OP::2MUL`;
+OP::7MUL = `OP_DUP OP::6MUL OP_ADD`;
+OP::8MUL = `OP::4MUL OP::2MUL`;
+OP::9MUL = OP::3MUL*2;
+
+// Static lookup table by index
+// For example: `3 lookup([10,11,12,13,14])` will leave `13` on the stack
+// stack in: <index>, stack out: <values[index]>
+fn lookup($values) = `
+  OP_TOALTSTACK // keep index number aside
+  reverse($values) // reversed so that index 0 gets the first element
+  OP_FROMALTSTACK OP_PICK // get index lookup result
+  OP_TOALTSTACK // keep lookup result aside
+  OP_2DROP*(len($values)/2) OP_DROP*(len($values)%2) // cleanup table
+  OP_FROMALTSTACK
+`;
+// h/t https://github.com/coins/bitcoin-scripts/
+
 //
 // Script Looping utilities
 //

src/stdlib/stdlib.minsc

@@ -32,6 +32,7 @@ fn some($arr, $fn)  = foldUntil($arr, false, |$acc, $el| if $fn($el)  then true:
 fn every($arr, $fn) = !some($arr, |$el| !$fn($el));
 
 fn contains($arr, $needle) = some($arr, |$el| $el == $needle);
+fn reverse($arr) = map(keys($arr), |$i| $arr.(len($arr)-$i-1));
 
 fn concat($xs) = reduce($xs, |$acc, $x| $acc + $x);
 sum = concat; // works for arrays, bytes, strings and numbers