From 018915b1a833717c0e5cc90db66b9540c90ea37e Mon Sep 17 00:00:00 2001 From: Miguel Dingli Date: Sat, 29 Aug 2020 12:13:36 +0200 Subject: [PATCH] Fix ApproxRoot Infinite Looping (#7140) Added a maximum number of iterations (100) to ApproxRoot (and ApproxSqrt) to serve as a hard limit on the number of iterations that the algorithm performs. If the answer converges before the maximum iterations, it returns immediately. Otherwise, if the answer never converges enough to satisfy the primary loop condition, the looping stops after a hundred iterations. Closes: #7038 Co-authored-by: Alessio Treglia --- CHANGELOG.md | 1 + types/decimal.go | 9 +++++++-- types/decimal_test.go | 21 ++++++++++++++------- 3 files changed, 22 insertions(+), 9 deletions(-) diff --git a/CHANGELOG.md b/CHANGELOG.md index 46b7dcf58..dd2117141 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -186,6 +186,7 @@ be used to retrieve the actual proposal `Content`. Also the `NewMsgSubmitProposa ### Bug Fixes +* (types) [\#7038](https://github.com/cosmos/cosmos-sdk/issues/7038) Fix infinite looping of `ApproxRoot` by including a hard-coded maximum iterations limit of 100. * (simulation) [\#7129](https://github.com/cosmos/cosmos-sdk/issues/7129) Fix support for custom `Account` and key types on auth's simulation. * (types) [\#7084](https://github.com/cosmos/cosmos-sdk/pull/7084) Fix panic when calling `BigInt()` on an uninitialized `Int`. * (x/bank) [\#6536](https://github.com/cosmos/cosmos-sdk/pull/6536) Fix bug in `WriteGeneratedTxResponse` function used by multiple diff --git a/types/decimal.go b/types/decimal.go index a6ef29a03..17b53ac56 100644 --- a/types/decimal.go +++ b/types/decimal.go @@ -18,13 +18,16 @@ type Dec struct { i *big.Int } -// number of decimal places const ( + // number of decimal places Precision = 18 // bytes required to represent the above precision // Ceiling[Log2[999 999 999 999 999 999]] DecimalPrecisionBits = 60 + + // max number of iterations in ApproxRoot function + maxApproxRootIterations = 100 ) var ( @@ -339,6 +342,8 @@ func (d Dec) QuoInt64(i int64) Dec { // using Newton's method (where n is positive). The algorithm starts with some guess and // computes the sequence of improved guesses until an answer converges to an // approximate answer. It returns `|d|.ApproxRoot() * -1` if input is negative. +// A maximum number of 100 iterations is used a backup boundary condition for +// cases where the answer never converges enough to satisfy the main condition. func (d Dec) ApproxRoot(root uint64) (guess Dec, err error) { defer func() { if r := recover(); r != nil { @@ -366,7 +371,7 @@ func (d Dec) ApproxRoot(root uint64) (guess Dec, err error) { rootInt := NewIntFromUint64(root) guess, delta := OneDec(), OneDec() - for delta.Abs().GT(SmallestDec()) { + for iter := 0; delta.Abs().GT(SmallestDec()) && iter < maxApproxRootIterations; iter++ { prev := guess.Power(root - 1) if prev.IsZero() { prev = SmallestDec() diff --git a/types/decimal_test.go b/types/decimal_test.go index dd7c497d8..60f29872f 100644 --- a/types/decimal_test.go +++ b/types/decimal_test.go @@ -406,15 +406,22 @@ func TestApproxRoot(t *testing.T) { root uint64 expected Dec }{ - {OneDec(), 10, OneDec()}, // 1.0 ^ (0.1) => 1.0 - {NewDecWithPrec(25, 2), 2, NewDecWithPrec(5, 1)}, // 0.25 ^ (0.5) => 0.5 - {NewDecWithPrec(4, 2), 2, NewDecWithPrec(2, 1)}, // 0.04 => 0.2 - {NewDecFromInt(NewInt(27)), 3, NewDecFromInt(NewInt(3))}, // 27 ^ (1/3) => 3 - {NewDecFromInt(NewInt(-81)), 4, NewDecFromInt(NewInt(-3))}, // -81 ^ (0.25) => -3 - {NewDecFromInt(NewInt(2)), 2, NewDecWithPrec(1414213562373095049, 18)}, // 2 ^ (0.5) => 1.414213562373095049 - {NewDecWithPrec(1005, 3), 31536000, MustNewDecFromStr("1.000000000158153904")}, + {OneDec(), 10, OneDec()}, // 1.0 ^ (0.1) => 1.0 + {NewDecWithPrec(25, 2), 2, NewDecWithPrec(5, 1)}, // 0.25 ^ (0.5) => 0.5 + {NewDecWithPrec(4, 2), 2, NewDecWithPrec(2, 1)}, // 0.04 ^ (0.5) => 0.2 + {NewDecFromInt(NewInt(27)), 3, NewDecFromInt(NewInt(3))}, // 27 ^ (1/3) => 3 + {NewDecFromInt(NewInt(-81)), 4, NewDecFromInt(NewInt(-3))}, // -81 ^ (0.25) => -3 + {NewDecFromInt(NewInt(2)), 2, NewDecWithPrec(1414213562373095049, 18)}, // 2 ^ (0.5) => 1.414213562373095049 + {NewDecWithPrec(1005, 3), 31536000, MustNewDecFromStr("1.000000000158153904")}, // 1.005 ^ (1/31536000) ≈ 1.00000000016 + {SmallestDec(), 2, NewDecWithPrec(1, 9)}, // 1e-18 ^ (0.5) => 1e-9 + {SmallestDec(), 3, MustNewDecFromStr("0.000000999999999997")}, // 1e-18 ^ (1/3) => 1e-6 + {NewDecWithPrec(1, 8), 3, MustNewDecFromStr("0.002154434690031900")}, // 1e-8 ^ (1/3) ≈ 0.00215443469 } + // In the case of 1e-8 ^ (1/3), the result repeats every 5 iterations starting from iteration 24 + // (i.e. 24, 29, 34, ... give the same result) and never converges enough. The maximum number of + // iterations (100) causes the result at iteration 100 to be returned, regardless of convergence. + for i, tc := range testCases { res, err := tc.input.ApproxRoot(tc.root) require.NoError(t, err)