Quick Circuit Resistance Calculator

resistance.py

# SOURCE: http://elonen.iki.fi/code/misc-notes/python-gaussj/
def gauss_jordan(m, eps = 1.0/(10**10)):
  """Puts given matrix (2D array) into the Reduced Row Echelon Form.
     Returns True if successful, False if 'm' is singular.
     NOTE: make sure all the matrix items support fractions! Int matrix will NOT work!
     Written by Jarno Elonen in April 2005, released into Public Domain"""
  (h, w) = (len(m), len(m[0]))
  for y in range(0,h):
    maxrow = y
    for y2 in range(y+1, h):    # Find max pivot
      if abs(m[y2][y]) > abs(m[maxrow][y]):
        maxrow = y2
    (m[y], m[maxrow]) = (m[maxrow], m[y])
    if abs(m[y][y]) <= eps:     # Singular?
      return False
    for y2 in range(y+1, h):    # Eliminate column y
      c = m[y2][y] / m[y][y]
      for x in range(y, w):
        m[y2][x] -= m[y][x] * c
  for y in range(h-1, 0-1, -1): # Backsubstitute
    c  = m[y][y]
    for y2 in range(0,y):
      for x in range(w-1, y-1, -1):
        m[y2][x] -=  m[y][x] * m[y2][y] / c
    m[y][y] /= c
    for x in range(h, w):       # Normalize row y
      m[y][x] /= c
  return True


def preprocess(edges, V):
    lists = []
    for i in range(V): lists.append([])
    for fro, to, res in edges:
        lists[fro].append((to, res))
        lists[to].append((fro, res))
    return lists

def make_eqns(lists, node1, node2):
    coeffs = []
    Vars = len(lists)
    for i, l in enumerate(lists):
        cs = [0.0]*Vars
        cs[i] = sum(1/b for (a, b) in l)
        for other, res in l:
            cs[other] -= 1.0/res
        coeffs.append(cs)
    rhs = [0] * Vars
    rhs[node1] = 1.0
    rhs[node2] = -1.0
    n = max(node1, node2)
    coeffs = [c[:n] + c[n + 1:] for c in coeffs]
    return coeffs[:-1], rhs[:-1]

def calculate_resistance(nodes, edges, source, destination):
    src = source
    dst = destination
    ls = preprocess(edges, nodes)
    a, b = make_eqns(ls, src, dst)
    M = [a[i] + [b[i]] for i in range(len(a))]
    gauss_jordan(M)
    return abs(M[min(src, dst)][-1])

if __name__ == '__main__':
    print calculate_resistance(4, [(0, 1, 1.0), (0, 2, 1.0), (0, 3, 1.0), (1, 3, 1.0), (2, 3, 1.0)], 0, 3)