Add exact_table_cover and parse_tile_exact functions for more specifi…

…c tile parsing (#237)

apycula/fuse_h4x.py

@@ -217,6 +217,67 @@ def parse_tile(d, ttyp, tile):
 
     return res
 
+def parse_tile_exact(d, ttyp, tile, fuse_loc=True):
+    w = d[ttyp]['width']
+    h = d[ttyp]['height']
+    res = {}
+    for start, table in [(2, 'shortval'), (2, 'wire'), (16, 'longval'),
+                         (1, 'longfuse'), (0, 'const')]:
+        if table in d[ttyp]: # skip missing entries
+            for subtyp, tablerows in d[ttyp][table].items():
+                pos_items, neg_items = {}, {}
+                active_rows = []
+                for row in tablerows:
+                    if row[0] > 0:
+                        row_fuses  = [fuse for fuse in row[start:] if fuse >= 0]
+                        locs = [fuse_lookup(d,ttyp, fuse) for fuse in row_fuses]
+                        test = [tile[loc[0]][loc[1]] == 1 for loc in locs]
+                        if all(test):
+                            full_row = row[:start]
+                            full_row.extend(row_fuses)
+                            active_rows.append(full_row)
+
+                # report fuse locations
+                if (active_rows):
+                    exact_cover = exact_table_cover(active_rows, start, table)
+                    if fuse_loc:
+                        for cover_row in exact_cover:
+                            cover_row[start:] = [fuse_lookup(d, ttyp, fuse) for fuse in cover_row[start:]]
+
+                    res.setdefault(table, {})[subtyp] = exact_cover
+    return res
+
+
+def exact_table_cover(t_rows, start, table=None):
+    try:
+        import xcover
+    except:
+        raise ModuleNotFoundError ("The xcover package needs to be installed to use the exact_cover function.\
+                                    \nYou may install it via pip: `pip install xcover`")
+
+    row_fuses = [set ([fuse for fuse in row[start:] if fuse!=-1]) for row in t_rows]
+    primary = set()
+    for row in row_fuses:
+        primary.update(row)
+    secondary = set()
+
+    # Enforce that every destination node has a single source
+    if table == 'wire':
+        for id, row in enumerate(t_rows):
+            # Casting the wire_id to a string ensures that it doesn't conflict with fuse_ids 
+            row_fuses[id].add(str(row[1]))
+            secondary.add(str(row[1]))
+
+    g = xcover.covers(row_fuses, primary=primary, secondary=secondary, colored=False)
+    if g:
+        for r in g:
+            #g is an iterator, so this is just a hack to return the first solution.
+            #A future commit might introduce a heuristic for determining what solution is most plausible
+            #where there are multiple solutions
+            return [t_rows[idx] for idx in r]
+    else:
+        return []
+
 def scan_fuses(d, ttyp, tile):
     w = d[ttyp]['width']
     h = d[ttyp]['height']