chain matrix product

# given matrix A1, A2, ..., An
# the sizes of them are m0*m1, m1*m2, ..., m(n-1) * mn
# in general, a matrix with size m*n  multipling a matrix with size of n*p will need mnp operations.
# let cost(i,j) be the minimum cost of prodcut Ai to Aj
# the size of product Ai to Aj is (i-1, j) 
# then cost(i,j) = min[cost(i,k) + cost(k+1, j) + m(i-1)*m(k)*m(j)]
# for any i, cost(i,i) = 0


def chain_matrix_product(shapes):
    """
    shapes : [(6,3), (3,1), (1,3), (3,8)]
    """
    cost_cache = {}
    # initial value
    for i in range(len(shapes)):
        for j in range(i, len(shapes)):
            cost_cache[(i,j)] = (0, [])
    
    for s in range(1, len(shapes)): # start from upper diagnal
        for i in range(len(shapes) - s):
            j = i + s
            min_val = float("inf")
            min_k = None
            for k in range(i,j):
                val = cost_cache[(i,k)][0]+cost_cache[(k+1,j)][0] + shapes[i][0]*shapes[k][1]*shapes[j][1]
                if val < min_val:
                    min_val = val
                    min_k = k
                
            partitions = [(i,min_k,j)] + cost_cache[(i,min_k)][1] + cost_cache[(min_k+1,j)][1]
            cost_cache[(i,j)] = (min_val, partitions)
    return cost_cache[(0, len(shapes)-1)]
if __name__ == "__main__":
    matrix = [(6,3), (3,1), (1,3), (3,8)]
    print(chain_matrix_product(matrix))

complexity O(n^3)

result (90, [(0,1,3),(0,0,1),(2,2,3)]

(i,k,j) partition is (i->k) (k+1 -> j)

[[(6,3)]*[(3,1)]]*[[(1,3)]*[(3,8)]]

matrix[0-0] matrix[1-1] => matrix[0-1]

=> matrix[0-1] * matrix[2-3]

matrix[2,2] matrix[3-3]=> matrix[2-3]