solution to problem 551 from project euler (#1164)

* solution to problem 551 from project euler

* renamed variables, and added more comments to improve readabilty

project_euler/problem_551/sol1.py

@@ -0,0 +1,204 @@
+"""
+Sum of digits sequence
+Problem 551
+
+Let a(0), a(1),... be an interger sequence defined by:
+     a(0) = 1
+     for n >= 1, a(n) is the sum of the digits of all preceding terms
+
+The sequence starts with 1, 1, 2, 4, 8, ...
+You are given a(10^6) = 31054319.
+
+Find a(10^15)
+"""
+
+ks = [k for k in range(2, 20+1)]
+base = [10 ** k for k in range(ks[-1] + 1)]
+memo = {}
+
+
+def next_term(a_i, k, i, n):
+    """
+    Calculates and updates a_i in-place to either the n-th term or the
+    smallest term for which c > 10^k when the terms are written in the form:
+            a(i) = b * 10^k + c
+
+    For any a(i), if digitsum(b) and c have the same value, the difference
+    between subsequent terms will be the same until c >= 10^k.  This difference
+    is cached to greatly speed up the computation.
+
+    Arguments:
+    a_i -- array of digits starting from the one's place that represent 
+           the i-th term in the sequence
+    k --  k when terms are written in the from a(i) = b*10^k + c.
+          Term are calulcated until c > 10^k or the n-th term is reached.
+    i -- position along the sequence
+    n -- term to caluclate up to if k is large enough
+
+    Return: a tuple of difference between ending term and starting term, and
+    the number of terms calculated. ex. if starting term is a_0=1, and
+    ending term is a_10=62, then (61, 9) is returned.
+    """
+    # ds_b - digitsum(b)
+    ds_b = 0
+    for j in range(k, len(a_i)):
+        ds_b += a_i[j]
+    c = 0
+    for j in range(min(len(a_i), k)):
+        c += a_i[j] * base[j]
+
+    diff, dn = 0, 0
+    max_dn = n - i
+
+    sub_memo = memo.get(ds_b)
+
+    if sub_memo != None:
+        jumps = sub_memo.get(c)
+
+        if jumps != None and len(jumps) > 0:
+            # find and make the largest jump without going over
+            max_jump = -1
+            for _k in range(len(jumps) - 1, -1, -1):
+                if jumps[_k][2] <= k and jumps[_k][1] <= max_dn:
+                    max_jump = _k
+                    break
+
+            if max_jump >= 0:
+                diff, dn, _kk = jumps[max_jump]
+                # since the difference between jumps is cached, add c
+                new_c = diff + c
+                for j in range(min(k, len(a_i))):
+                    new_c, a_i[j] = divmod(new_c, 10)
+                if new_c > 0:
+                    add(a_i, k, new_c)
+
+        else:
+            sub_memo[c] = []
+    else:
+        sub_memo = {c: []}
+        memo[ds_b] = sub_memo
+
+    if dn >= max_dn or c + diff >= base[k]:
+        return diff, dn
+
+    if k > ks[0]:
+        while True:
+            # keep doing smaller jumps
+            _diff, terms_jumped = next_term(a_i, k - 1, i + dn, n)
+            diff += _diff
+            dn += terms_jumped
+
+            if dn >= max_dn or c + diff >= base[k]:
+                break
+    else:
+        # would be too small a jump, just compute sequential terms instead
+        _diff, terms_jumped = compute(a_i, k, i + dn, n)
+        diff += _diff
+        dn += terms_jumped
+
+    jumps = sub_memo[c]
+
+    # keep jumps sorted by # of terms skipped
+    j = 0
+    while j < len(jumps):
+        if jumps[j][1] > dn:
+            break
+        j += 1
+
+    # cache the jump for this value digitsum(b) and c
+    sub_memo[c].insert(j, (diff, dn, k))
+    return (diff, dn)
+
+
+def compute(a_i, k, i, n):
+    """
+    same as next_term(a_i, k, i, n) but computes terms without memoizing results.
+    """
+    if i >= n:
+        return 0, i
+    if k > len(a_i):
+        a_i.extend([0 for _ in range(k - len(a_i))])
+
+    # note: a_i -> b * 10^k + c
+    # ds_b -> digitsum(b)
+    # ds_c -> digitsum(c)
+    start_i = i
+    ds_b, ds_c, diff = 0, 0, 0
+    for j in range(len(a_i)):
+        if j >= k:
+            ds_b += a_i[j]
+        else:
+            ds_c += a_i[j]
+
+    while i < n:
+        i += 1
+        addend = ds_c + ds_b
+        diff += addend
+        ds_c = 0
+        for j in range(k):
+            s = a_i[j] + addend
+            addend, a_i[j] = divmod(s, 10)
+
+            ds_c += a_i[j]
+
+        if addend > 0:
+            break
+
+    if addend > 0:
+        add(a_i, k, addend)
+    return diff, i - start_i
+
+
+def add(digits, k, addend):
+    """
+    adds addend to digit array given in digits
+    starting at index k
+    """
+    for j in range(k, len(digits)):
+        s = digits[j] + addend
+        if s >= 10:
+            quotient, digits[j] = divmod(s, 10)
+            addend = addend // 10 + quotient
+        else:
+            digits[j] = s
+            addend = addend // 10
+
+        if addend == 0:
+            break
+
+    while addend > 0:
+        addend, digit = divmod(addend, 10)
+        digits.append(digit)
+
+
+def solution(n):
+    """
+    returns n-th term of sequence
+
+    >>> solution(10)
+    62
+
+    >>> solution(10**6)
+    31054319
+
+    >>> solution(10**15)
+    73597483551591773
+    """
+
+    digits = [1]
+    i = 1
+    dn = 0
+    while True:
+        diff, terms_jumped = next_term(digits, 20, i + dn, n)
+        dn += terms_jumped
+        if dn == n - i:
+            break
+
+    a_n = 0
+    for j in range(len(digits)):
+        a_n += digits[j] * 10 ** j
+    return a_n
+
+
+if __name__ == "__main__":
+    print(solution(10 ** 15))