Add STV

README_STV.md

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+The format for all election data is (each element on a new line):
+* Number of Candidates
+* 1, Candidate Name
+* 2, Candidate Name
+* ...
+* Number of Voters, Sum of Vote Count, Number of Unique Orders
+* count, preference list. (12,1,2,{3,4}). A strict ordering is indicated by a comma (,) and elements that are indifferent are grouped with a ({ }).
+* count, preference list. (12,1,2,3,4). A strict ordering is indicated by a comma (,) and elements that are indifferent are grouped with a ({ }).
+* ...
+
+The above is based on preflib's format. [Visit them for more info](http://www.preflib.org/data/format.php#election-data)  
+
+Demo usage code is in [mechanismSTVRunner.py](./prefpy/mechanismSTVRunner.py)  
+Sample input are in [prefpy/tests](./tests)
+
+There are two main mechanisms that implement STV: MechanismSTV and MechanismSTVAll.
+
+* MechanismSTV has several child mechanisms, each implementing its own tie-breaking rule. Tie-breaking is done to determine which candidate is eliminated.
+  * MechanismSTVForward implements forward tie-breaking
+    * The candidate with the least votes in the earliest round is eliminated
+  * MechanismSTVBackward implements backward tie-breaking
+    * The candidate with the leave votes at the latest non-tied round is eliminated
+  * MechanismSTVBorda implements borda tie-breaking
+    * The candidate with the lowest Borda score is eliminated
+  * MechanismSTVCoombs implements coombs tie-breaking
+    * The candidate with the most last-place votes is eliminated
+* MechanismSTVAll determines all possible winners given all possible tie-breaking schemes

prefpy/mechanismSTVRunner.py

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+import sys
+from prefpy import io
+from prefpy import mechanismSTV
+from .preference import Preference
+from .profile import Profile
+
+def electionFileToProfile(file):
+    inputFile = open(sys.argv[1], 'r')
+    candMap, rankMaps, rankMapsCounts, numVoters = io.read_election_file(inputFile)
+
+    preferences = []
+    # ranking: [dict<cand,pos in order>, freq of ranking]
+    for ranking in zip(rankMaps,rankMapsCounts):
+        cands = list(ranking[0].keys())
+        wmg = {}
+        for i in range(len(cands)):
+            for j in range(i+1, len(cands)):
+                cand1, cand2 = cands[i], cands[j]
+                if cand1 not in wmg:
+                    wmg[cand1] = {}
+                if cand2 not in wmg:
+                    wmg[cand2] = {}
+                if ranking[0][cand1] < ranking[0][cand2]:
+                    wmg[cand1][cand2] = 1
+                    wmg[cand2][cand1] = -1
+                elif ranking[0][cand1] > ranking[0][cand2]:
+                    wmg[cand1][cand2] = -1
+                    wmg[cand2][cand1] = 1
+                else:
+                    wmg[cand1][cand2] = 0
+                    wmg[cand2][cand1] = 0
+        preferences.append(Preference(wmg, ranking[1]))
+    return Profile(candMap, preferences)
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("Usage: python3 -m prefpy.mechanismSTVRunner tests/<any test file>\n",
+              ''' input file follows the election data format described at (http://www.preflib.org/data/format.php#election-data):
+    * <number of candidates>
+    * <candidate number; e.g. 1>,<candidate name; e.g. apple>
+    * ... //until all candidates listed
+    * <total number of voters>,<sum of vote count;usually same as total number of votes>,<number of unique rankings>
+    * <number of votes with this ranking>,<ranking;e.g. 1,2,3,4>
+    * ... //until all unique rankings are listed''')
+        exit()
+    profile = electionFileToProfile(sys.argv[1])
+
+    import time    
+
+    t0 = time.time()
+    stv = mechanismSTV.MechanismSTVForward()
+    winners = stv.getWinners(profile)
+    t1 = (time.time() - t0) * 1000000.0
+    print("\n\n")
+    print("Using STV with forward tie breaking")
+    print(winners)
+    print("Took %.1fus" % t1)
+    print("\n" + "=" * 80)
+
+    t0 = time.time()
+    stv = mechanismSTV.MechanismSTVBackward()
+    winners = stv.getWinners(profile)
+    t2 = (time.time() - t0) * 1000000.0
+    print("Using STV with backward tie breaking")
+    print(winners)
+    print("Took %.1fus" % t2)
+    print("\n" + "=" * 80)
+
+    t0 = time.time()
+    stv = mechanismSTV.MechanismSTVBorda()
+    winners = stv.getWinners(profile)
+    t3 = (time.time() - t0) * 1000000.0
+    print("Using STV with Borda tie breaking")
+    print(winners)
+    print("Took %.1fus" % t3)
+    print("\n" + "=" * 80)
+
+    t0 = time.time()
+    stv = mechanismSTV.MechanismSTVCoombs()
+    winners = stv.getWinners(profile)
+    t4 = (time.time() - t0) * 1000000.0
+    print("Using STV with Coombs tie breaking")
+    print(winners)
+    print("Took %.1fus" % t4)
+    print("\n" + "=" * 80)
+
+    t0 = time.time()
+    stv = mechanismSTV.MechanismSTVAll()
+    winners = stv.getWinners(profile)
+    t5 = (time.time() - t0) * 1000000.0
+    print("Using STV - All possible winners")
+    print(winners)
+    print("Took %.1fus" % t5)
+    print("\n" + "=" * 80)
+    print("\n\n")

tests/stvTestInput01

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+4
+1,a
+2,b
+3,c
+4,d
+26,26,4
+10,1,2,3,4
+7,4,1,2,3
+6,3,4,1,2
+3,2,3,4,1

tests/stvTestInput02

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+4
+1,a
+2,b
+3,c
+4,d
+24,24,4
+10,1,2,3,4
+7,4,1,2,3
+6,3,4,1,2
+1,2,3,4,1

tests/stvTestInput03

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+4
+1,a
+2,b
+3,c
+4,d
+20,20,5
+10,1,2,3,4
+5,4,3,2,1
+3,3,4,3,1
+1,1,3,2,4
+1,3,2,{1,4}

tests/stvTestInput04

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+4
+1,a
+2,b
+3,c
+4,d
+18,18,6
+6,1,2,3,4
+4,2,1,4,3
+1,3,1,4,2
+3,3,2,4,1
+2,4,2,1,3
+2,4,1,3,2

tests/stvTestInput05

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+4
+1,a
+2,b
+3,c
+4,d
+20,20,6
+6,1,2,3,4
+6,2,1,4,3
+2,3,2,4,1
+2,3,4,1,2
+3,4,2,1,3
+1,4,3,1,2

tests/stvTestInput06

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+4
+1,a
+2,b
+3,c
+4,d
+22,22,5
+6,1,2,3,4
+9,2,3,1,4
+3,3,1,2,4
+2,4,1,2,3
+2,4,2,1,3

tests/stvTestInput07

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+4
+1,a
+2,b
+3,c
+4,d
+18,18,5
+7,1,2,3,4
+7,2,3,1,4
+1,3,2,1,4
+2,3,1,2,4
+1,4,2,1,3

tests/stvTestInput08

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+7
+1,a
+2,b
+3,c
+4,d
+5,e
+6,f
+7,g
+228,228,8
+43,1,2,3,4,5,7,6
+11,2,5,1,4,3,6,7
+33,3,1,5,4,2,6,7
+21,4,3,2,1,5,7,6
+11,5,4,2,1,3,6,7
+1,4,1,2,5,3,6,7
+54,6,3,1,2,3,5,7
+54,7,3,6,1,2,3,4