diff options
Diffstat (limited to 'absl/strings/cord.h')
| -rw-r--r-- | absl/strings/cord.h | 38 |
1 files changed, 10 insertions, 28 deletions
diff --git a/absl/strings/cord.h b/absl/strings/cord.h index 3941f19c3451..eb236e50e22e 100644 --- a/absl/strings/cord.h +++ b/absl/strings/cord.h @@ -72,39 +72,21 @@ namespace cord_internal { // It's expensive to keep a tree perfectly balanced, so instead we keep trees // approximately balanced. A tree node N of depth D(N) that contains a string // of L(N) characters is considered balanced if L >= Fibonacci(D + 2). -// The "+ 2" is used to ensure that every leaf node contains at least one -// character. Here we presume that +// The "+ 2" is used to ensure that every balanced leaf node contains at least +// one character. Here we presume that // Fibonacci(0) = 0 // Fibonacci(1) = 1 // Fibonacci(2) = 1 // Fibonacci(3) = 2 // ... -// -// Fibonacci numbers are convenient because it means when two balanced trees of -// the same depth are made the children of a new node, the resulting tree is -// guaranteed to also be balanced: -// -// -// L(left) >= Fibonacci(D(left) + 2) -// L(right) >= Fibonacci(D(right) + 2) -// -// L(left) + L(right) >= Fibonacci(D(left) + 2) + Fibonacci(D(right) + 2) -// L(left) + L(right) == L(new_tree) -// -// L(new_tree) >= 2 * Fibonacci(D(child) + 2) -// D(child) == D(new_tree) - 1 -// -// L(new_tree) >= 2 * Fibonacci(D(new_tree) + 1) -// 2 * Fibonacci(N) >= Fibonacci(N + 1) -// -// L(new_tree) >= Fibonacci(D(new_tree) + 2) -// -// -// The 93rd Fibonacci number is the largest Fibonacci number that can be -// represented in 64 bits, so the size of a balanced Cord of depth 92 is too big -// for an unsigned 64 bit integer to hold. Therefore we can safely assume that -// the maximum depth of a Cord is 91. -constexpr size_t MaxCordDepth() { return 91; } +// The algorithm is based on paper by Hans Boehm et al: +// https://www.cs.rit.edu/usr/local/pub/jeh/courses/QUARTERS/FP/Labs/CedarRope/rope-paper.pdf +// In this paper authors shows that rebalancing based on cord forest of already +// balanced subtrees can be proven to never produce tree of depth larger than +// largest Fibonacci number representable in the same integral type as cord size +// For 64 bit integers this is the 93rd Fibonacci number. For 32 bit integrals +// this is 47th Fibonacci number. +constexpr size_t MaxCordDepth() { return sizeof(size_t) == 8 ? 93 : 47; } // This class models fixed max size stack of CordRep pointers. // The elements are being pushed back and popped from the back. |