Saturday, July 25

value-overflow

daily challenge16

today's brief

August 15, 2010. Someone broadcasts a Bitcoin transaction that creates 184 billion coins out of thin air, far more than will ever exist. The exploit was two outputs with enormous amounts chosen so their sum overflowed the integer used to total them, wrapped around to a small number, and slipped straight past the check that outputs cannot exceed inputs. Satoshi patched it within hours and the chain was forked to erase it. It is the only time coins were ever conjured from a bug. Total the outputs without letting them wrap.

Objective

Return whether two outputs are valid: their sum must not overflow a signed 32-bit total.

Steps

You get two output amounts, a and b, both non-negative.
A signed 32-bit integer tops out at 2147483647.
Return 1 if a + b stays at or below that maximum, 0 if the sum would overflow (the money printing case).
Check for the overflow before you add, not after: a + b in an int would already have wrapped.

examples

$ sum_ok(1, 1)

$ sum_ok(2000000000, 2000000000)

$ sum_ok(2147483647, 0)

Constraints

Expected files

outputs.c

Allowed functions

None. Write every helper yourself.

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Solutions

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