From 37135dc7daae95715a5341c72646beb147cc96af Mon Sep 17 00:00:00 2001
From: Gard Spreemann <gard.spreemann@epfl.ch>
Date: Wed, 24 Oct 2018 13:43:43 +0200
Subject: Near drop-in Python replacement for small problems.

---
 lappy.py | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)
 create mode 100755 lappy.py

diff --git a/lappy.py b/lappy.py
new file mode 100755
index 0000000..5d50372
--- /dev/null
+++ b/lappy.py
@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+
+import scipy.sparse.linalg
+import sys
+import numpy as np
+import petsc
+
+def print_usage():
+    sys.stderr.write("Usage:\n")
+    sys.stderr.write("%s --laplacian|-l file --number|-n n --vals file --vecs file [-h|--help]\n" %(sys.argv[0]))
+
+def main():
+    n = 0
+    i = 1
+    while i < len(sys.argv):
+        arg = sys.argv[i]
+        if arg in ["-h", "--help"]:
+            print_usage()
+            exit(0)
+        elif arg in ["-l", "--laplacian"]:
+            if i + 1 < len(sys.argv) and sys.argv[i+1][0] != "-":
+                infile = sys.argv[i+1]
+                i += 1
+            else:
+                sys.stderr.write("Missing argument to --laplacian.\n")
+                print_usage()
+                exit(1)
+        elif arg in ["--vals"]:
+            if i + 1 < len(sys.argv) and sys.argv[i+1][0] != "-":
+                outfile_vals = sys.argv[i+1]
+                i += 1
+        elif arg in ["--vecs"]:
+            if i + 1 < len(sys.argv) and sys.argv[i+1][0] != "-":
+                outfile_vecs = sys.argv[i+1]
+                i += 1
+        elif arg in ["-n", "--number"]:
+            if i + 1 < len(sys.argv):
+                try:
+                    n = int(sys.argv[i+1])
+                    i += 1
+                except ValueError as err:
+                    sys.stderr.write("Invalid argument to --number.\n")
+                    print_usage()
+                    exit(1)
+        else:
+            sys.stderr.write("Unknown option %s.\n" %(arg))
+            print_usage()
+            exit(1)
+        i += 1
+
+    if infile == "" or outfile_vals == "" or outfile_vecs == "":
+        sys.stderr.write("Need a Laplacian and both input files.\n")
+        print_usage()
+        exit(1)
+    if n <= 0:
+        sys.stderr.write("The --number parameter is mandatory and must be >0.\n")
+        print_usage()
+        exit(1)
+            
+    L = petsc.load(infile)
+    print("Laplacian is size %d." %(L.shape[0]))
+    
+    (vals, vecs) = scipy.sparse.linalg.eigsh(L, k=min(n, L.shape[0]), M=None, sigma=None, which="SM", v0=None, ncv=None, maxiter=None, tol=0, return_eigenvectors=True, Minv=None, OPinv=None, mode="normal")
+
+    print("Have %d eigenpairs." %(len(vals)))
+    cutoff = 1e-6 # Arbitrary
+    estimated_betti = 0
+    with open(outfile_vals, "w") as f:
+        for i in range(0, len(vals)):
+            f.write("%f\n" %(vals[i]))
+            print("%d: %.20f" %(i, vals[i]))
+            if np.abs(vals[i]) < cutoff:
+                estimated_betti += 1
+
+    print("Estimated Betti number: %d." %(estimated_betti))
+
+    np.savetxt(outfile_vecs, vecs.T)
+
+if  __name__ == "__main__":
+    main()
-- 
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