The Chant-Stack of the Ystrevel Golems

Beneath the ruined foundry at Ystrevel, the old clay golems still wake when an adventurer reads a chant to them. Each golem keeps a stack of whole numbers in its chest and obeys the chant one token at a time. The Foundry Wardens of Ystrevel will pay anyone who can say, before a chant is spoken, exactly what a golem will hold when it falls silent, or exactly how it will fail.

Write golemChant(script). script is a string. Return either a list of integers (the final stack, bottom first) or one of the four error strings listed below.

Tokens. Split script on whitespace (spaces, tabs, newlines). Runs of whitespace and leading or trailing whitespace never produce empty tokens. Each token must be one of these: - an integer: an optional - followed by one or more digits 0-9, such as 7, -12, 007 or -0. Nothing else is an integer, so +5, - and 3.5 are not. - a word: one of add sub mul div dup drop swap if else end, matched case-insensitively. ADD and Add both mean add.

Structure. Every if must be closed by a later matching end, and they nest like brackets. Between an if and its matching end, at that same nesting level, there may be at most one else. The structure is malformed if an else or end appears with no open if, if an if has two elses at its own level, or if an if is never closed.

Running. Start with an empty stack and process tokens from left to right: - An integer is pushed. - add, sub, mul and div pop b (the top value) and then pop a. They push a + b, a - b, a * b, or a / b truncated toward zero, so -7 / 2 gives -3. - dup pushes a copy of the top value. drop pops and discards the top value. swap exchanges the top two values. - if pops the top value. - If that value is nonzero, the tokens after the if run up to its else (or up to its end if it has no else), and running then continues after its end. - If that value is zero, running continues just after its else. If it has no else, running continues just after its end. - When running reaches an else, it jumps to just after that else's matching end. When running reaches an end, it does nothing.

Tokens in a branch that is skipped are not run.

Result. These checks are made in order: 1. If any token is neither an integer nor a word, return "unknown". This applies to every token in the script, including tokens in branches that would be skipped. 2. Otherwise, if the structure is malformed, return "malformed". 3. Otherwise run the chant. Running stops at the first error: - If a token needs more values than the stack holds, return "underflow". if, dup and drop need 1 value. add, sub, mul, div and swap need 2. - If a div has enough values but b is 0, return "division by zero". 4. If the chant finishes, return the stack as a list, bottom first. An empty or all-whitespace script returns [].

No test produces a value outside the range of a 32-bit signed integer.