The Sigil-Tablet of Ferrowind Caldera

On the floor of Ferrowind Caldera, a row of basalt columns stands shoulder to shoulder, running west to east. Each column is exactly one stride wide, and no gap separates neighbours. The Wardwrights' Guild has posted a commission. They will carve one rectangular warding sigil across the faces of a contiguous run of columns. The sigil's base rests on the caldera floor, and its top may not rise above the shortest column in that run. The Guild pays by area, so they want the largest sigil the row can hold.

Write largestSigil(heights).

• heights is a list of non-negative integers. The value at each position is the height of that column, in strides, in west-to-east order. The list may be empty, and a height may be 0.
• For a non-empty contiguous run from index i to index j inclusive, the area of the sigil is (j - i + 1) * min(heights[i..j]).
• Return the largest such area over all non-empty contiguous runs, as an integer.
• If heights is empty, return 0.