Day 13 python by preng

days/day-13/io/preng.input

@@ -0,0 +1,2 @@
+1008141
+17,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,x,523,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,13,19,x,x,x,23,x,x,x,x,x,x,x,787,x,x,x,x,x,37,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,29

days/day-13/io/preng.output

@@ -0,0 +1,2 @@
+4722
+825305207525452

days/day-13/solutions/day13.preng.py

@@ -0,0 +1,81 @@
+from functools import reduce
+from sys import stdin
+import math
+
+def get_stdin():
+    return [line.rstrip() for line in stdin]
+
+lines = get_stdin()
+
+# PART 1
+
+earliest = int(lines[0])
+buses = [int(n) for n in lines[1].split(",") if n != "x"]
+
+now = False
+t = earliest
+while not now:
+    for bus in buses:
+        if t % bus == 0:
+            #print("success at", t, "with", bus)
+            print((t-earliest) * bus)
+            now = True
+    t += 1
+
+
+# PART 2
+
+def busno(num):
+    if num == "x":
+        return -1
+    else:
+        return int(num)
+
+# Googled modulo equations and the Chinese theorem. Not fully understanding why this works.
+def mul_inv(a, b):
+    b0 = b
+    x0, x1 = 0, 1
+    if b == 1: return 1
+    while a > 1:
+        q = a // b
+        a, b = b, a%b
+        x0, x1 = x1 - q * x0, x0
+    if x1 < 0: x1 += b0
+    return x1
+
+def chinese_remainder(n, a):
+    sum = 0
+    prod = reduce(lambda a, b: a*b, n)
+    for n_i, a_i in zip(n, a):
+        p = prod // n_i
+        sum += a_i * mul_inv(p, n_i) * p
+    return sum % prod
+
+def find_earliest(buses):
+    modulos = [n for n in buses if n != -1]
+    deltas = []
+    for i in range(len(buses)):
+        if buses[i] != -1:
+            deltas.append(-i)
+    #print(modulos, deltas)
+    return(chinese_remainder(modulos, deltas))
+
+tests = [
+    [-1,-1,3,5],
+    [7,13,-1,-1,59,-1,31,19],
+    [17,-1,13,19],
+    [67,7,59,61],
+    [67,-1,7,59,61],
+    [67,7,-1,59,61],
+    [1789,37,47,1889]
+]
+
+# for t in tests:
+#     print(find_earliest(t), t)
+#     print("")
+# exit(0)
+
+buses = [busno(n) for n in lines[1].split(",")]
+
+#print(buses, len(buses))
+print(find_earliest(buses))

days/day-13/test.sh

@@ -3,10 +3,8 @@ set -euo pipefail
 
 D=$(dirname $(realpath $0))
 
-echo "--- Day 13: --- ????? --- ❌"
+echo "--- Day 13: --- Shuttle search ---"
 #$D/../../lang/go.sh     "$D/solutions/*.go"  "$D/io/*"
 #$D/../../lang/sml.sh    "$D/solutions/*.sml" "$D/io/*"
-#$D/../../lang/python.sh "$D/solutions/*.py"  "$D/io/*"
+$D/../../lang/python.sh "$D/solutions/*.py"  "$D/io/*"
 #$D/../../lang/deno.sh   "$D/solutions/*.ts"  "$D/io/*"
-
-exit 1337;