Description of solution

day05-2.py

@@ -4,6 +4,14 @@ helper = Helper(debug=True)
 load_input = helper.load_input
 debug = helper.debug
 
+# FUTURE ME: HERE'S THE KEY
+# You don't have to check every seed.
+# Instead, create ranges, and use the later steps to cut those ranges
+# into chunks. The smallest value in the smallest chunk after the last
+# step is the solution. The brute-force solution takes an enormous amount
+# of time; the chunk solution is still polynomial time, I think, but 
+# the n is much smaller (dozens rather than hundreds of millions).
+
 def within(target, start, end):
     if start == end:
         return target == start
@@ -11,6 +19,40 @@ def within(target, start, end):
         start, end = end, start
     return target >= start and end >= target
 
+def chunk_it(old_range, new_range):
+    o_start, o_end = old_range
+    n_start, n_end = new_range
+    debug(o_start, o_end, n_start, n_end)
+
+    # The new range is entirely outside the current range
+    if n_end < o_start or n_start > o_end:
+        debug("Entirely outside!")
+        return sorted([old_range, new_range], key=lambda n:n[0])
+    
+    # The new range is entirely within the current range
+    if n_start >= o_start and n_end <= o_end:
+        # We've already accounted for this range
+        debug("Already got it")
+        return [old_range]
+
+    # The new range starts lower and ends higher
+    if n_start <= o_start and n_end >= o_end:
+        # We can replace the old range with the new one
+        debug("Replacement!")
+        return [new_range]
+
+    # The new range starts lower but ends within
+    if n_start <= o_start and n_end >= o_start and n_end <= o_end:
+        debug("Starts before")
+        return [[n_start, o_end]]
+
+    # The new range starts within but ends higher
+    if n_end >= o_end and n_start >= o_start and n_start <= o_end:
+        debug("Ends after")
+        return [[o_start, n_end]]
+    
+    raise Exception(f"What? What? Nothing matched: {old_range}, {new_range}")
+
 def main():
     lines = load_input(5)
 
@@ -26,10 +68,29 @@ def main():
     }
     maps_keys = list(maps_dict.keys())
     maps_dict["seeds"] = [int(n) for n in lines[0].split()[1:]]
+    debug(f"Starting with {len(maps_dict['seeds'])//2} ranges")
     real_seeds = []
     i, j = 0, 1
     while j < len(maps_dict["seeds"]):
-        real_seeds.append((maps_dict["seeds"][i], maps_dict["seeds"][i] + maps_dict["seeds"][j]))
+        new_range = [maps_dict["seeds"][i], maps_dict["seeds"][i] + maps_dict["seeds"][j]]
+        if len(real_seeds) == 0:
+            real_seeds.append(new_range)
+        else:
+            new_real_seeds = []
+            for range in real_seeds:
+                add_ranges = chunk_it(range, new_range)
+                debug(add_ranges)
+                for r in add_ranges:
+                    if r not in new_real_seeds:
+                        new_real_seeds.append(r)
+            real_seeds = list(new_real_seeds)
+        i += 2
+        j += 2
+        debug(real_seeds, i, j)
+    debug(f"Found {len(real_seeds)} actual ranges")
+    real_seeds.sort(key=lambda n: n[0])
+    # debug(real_seeds)
+    return
         # new_start = maps_dict["seeds"][i]
         # new_end = new_start + maps_dict["seeds"][j]
         # debug(f"Adding seeds in range {new_start}-{new_end}")
@@ -37,8 +98,6 @@ def main():
         # debug(new_seeds)
         # real_seeds.append(new_seeds)
         # debug(real_seeds)
-        i += 2
-        j += 2
     seeds_list = [{"seed": seed[1]} for seed in real_seeds]
     
     current_key = ""