Source code in the zkl programming language

fcn waterCollected(walls){
     // compile max wall heights from left to right and right to left
     // then each pair is left/right wall of that cell.
     // Then the min of each wall pair == water height for that cell
   scanl(walls,(0).max)     // scan to right, f is max(0,a,b)
  .zipWith((0).MAX.min,     // f is MAX.min(a,b) == min(a,b)
           scanl(walls.reverse(),(0).max).reverse()) // right to left
     // now subtract the wall height from the water level and add 'em up
   .zipWith('-,walls).filter('>(0)).sum(0);
} 
fcn scanl(xs,f,i=0){ // aka reduce but save list of results
   xs.reduce('wrap(s,x,a){ s=f(s,x); a.append(s); s },i,ss:=List()); 
   ss
} // scanl((1,5,3,7,2),max,0) --> (1,5,5,7,7)

T( T(1, 5, 3, 7, 2), T(5, 3, 7, 2, 6, 4, 5, 9, 1, 2),
   T(2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1),
   T(5, 5, 5, 5), T(5, 6, 7, 8),T(8, 7, 7, 6),
   T(6, 7, 10, 7, 6) )
.pump(List, waterCollected).println();