day 15 with two solutions: slow and LUT approaches

15/a.py

@@ -0,0 +1,38 @@
+arr = [ int(x) for x in open('input').read().split(',') ]
+idx = 30000000
+
+def lookback(n, arr):
+    arr.reverse()
+    A = 0; c = 0
+    for x in arr:
+        c += 1
+        if x == n:
+            if A == 0:
+                A = c
+            else:
+                return c - A
+    return 0
+
+def a(arr, idx):
+    for t in range(len(arr),idx):
+        arr.append(lookback(arr[t-1], arr[:t]))
+    return arr[idx-1]
+
+# print(a(arr, idx))
+# OK the lookback approach is slow AF. Let's try arr different method!
+
+def lut_search(lut, n, idx):
+    if n in lut:
+        d = idx - lut[n]
+        lut[n] = idx
+        return d
+    else:
+        lut.update({ n: idx })
+        return 0
+def b(arr, idx):
+    lut = { arr[x]:x+1 for x in range(len(arr)) }
+    for t in range(len(arr),idx):
+        arr.append(lut_search(lut, arr[t-1], t))
+    return arr[idx-1] 
+
+print(b(arr, idx))