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Handle negative exponent and rename value/pow10 to numerator/denominator

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This commit is contained in:
Victor Zverovich
2019-10-13 11:36:21 -07:00
parent f7a5748fd3
commit 1cbc5fa6cb
4 changed files with 127 additions and 67 deletions
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174
include/fmt/format-inl.h
View File

@@ -502,21 +502,18 @@ class bigint {
friend struct formatter<bigint>;
void assign(uint64_t n) {
int num_bigits = 0;
do {
bigits_[num_bigits++] = n & ~bigit(0);
n >>= bigit_bits;
} while (n != 0);
bigits_.resize(num_bigits);
}
void subtract_bigits(int index, bigit other, bigit& borrow) {
auto result = static_cast<double_bigit>(bigits_[index]) - other - borrow;
bigits_[index] = static_cast<bigit>(result);
borrow = static_cast<bigit>(result >> (bigit_bits * 2 - 1));
}
void remove_leading_zeros() {
int num_bigits = static_cast<int>(bigits_.size()) - 1;
while (num_bigits > 0 && bigits_[num_bigits] == 0) --num_bigits;
bigits_.resize(num_bigits + 1);
}
// Computes *this -= other assuming aligned bigints and *this >= other.
void subtract_aligned(const bigint& other) {
FMT_ASSERT(other.exp_ >= exp_, "unaligned bigints");
@@ -528,18 +525,61 @@ class bigint {
subtract_bigits(i, other.bigits_[j], borrow);
}
while (borrow > 0) subtract_bigits(i, 0, borrow);
remove_leading_zeros();
}
void multiply(uint32_t value) {
const double_bigit wide_value = value;
bigit carry = 0;
for (size_t i = 0, n = bigits_.size(); i < n; ++i) {
double_bigit result = bigits_[i] * wide_value + carry;
bigits_[i] = static_cast<bigit>(result);
carry = static_cast<bigit>(result >> bigit_bits);
}
if (carry != 0) bigits_.push_back(carry);
}
void multiply(uint64_t value) {
const bigit mask = ~bigit(0);
const double_bigit lower = value & mask;
const double_bigit upper = value >> bigit_bits;
double_bigit carry = 0;
for (size_t i = 0, n = bigits_.size(); i < n; ++i) {
double_bigit result = bigits_[i] * lower + (carry & mask);
carry =
bigits_[i] * upper + (result >> bigit_bits) + (carry >> bigit_bits);
bigits_[i] = static_cast<bigit>(result);
}
while (carry != 0) {
bigits_.push_back(carry & mask);
carry >>= bigit_bits;
}
}
public:
bigint() : exp_(0) {}
template <typename Int> explicit bigint(Int n) : exp_(0) {
assign(uint32_or_64_or_128_t<Int>(to_unsigned(n)));
}
explicit bigint(uint64_t n) { assign(n); }
bigint(const bigint&) = delete;
void operator=(const bigint&) = delete;
void assign(const bigint& other) {
bigits_.resize(other.bigits_.size());
auto data = other.bigits_.data();
std::copy(data, data + other.bigits_.size(), bigits_.data());
exp_ = other.exp_;
}
void assign(uint64_t n) {
int num_bigits = 0;
do {
bigits_[num_bigits++] = n & ~bigit(0);
n >>= bigit_bits;
} while (n != 0);
bigits_.resize(num_bigits);
exp_ = 0;
}
int num_bigits() const { return static_cast<int>(bigits_.size()) + exp_; }
bigint& operator<<=(int shift) {
@@ -557,20 +597,9 @@ class bigint {
return *this;
}
bigint& operator*=(uint32_t value) {
assert(value > 0);
// Verify that the computation cannot overflow.
constexpr double_bigit max_bigit = max_value<bigit>();
constexpr double_bigit max_double_bigit = max_value<double_bigit>();
static_assert(max_bigit * max_bigit <= max_double_bigit - max_bigit, "");
bigit carry = 0;
const double_bigit wide_value = value;
for (size_t i = 0, n = bigits_.size(); i < n; ++i) {
double_bigit result = bigits_[i] * wide_value + carry;
bigits_[i] = static_cast<bigit>(result);
carry = static_cast<bigit>(result >> bigit_bits);
}
if (carry != 0) bigits_.push_back(carry);
template <typename Int> bigint& operator*=(Int value) {
FMT_ASSERT(value > 0, "");
multiply(uint32_or_64_or_128_t<Int>(value));
return *this;
}
@@ -659,11 +688,8 @@ class bigint {
bigits_[bigit_index] = static_cast<bigit>(sum);
sum >>= bits<bigit>::value;
}
// Remove leading zeros.
--num_result_bigits;
while (num_result_bigits > 0 && bigits_[num_result_bigits] == 0)
--num_result_bigits;
bigits_.resize(num_result_bigits + 1);
remove_leading_zeros();
exp_ *= 2;
}
@@ -907,7 +933,7 @@ template <int GRISU_VERSION> struct grisu_shortest_handler {
}
};
// Format value using a variation of the Fixed-Precision Positive Floating-Point
// Formats v using a variation of the Fixed-Precision Positive Floating-Point
// Printout ((FPP)^2) algorithm by Steele & White:
// http://kurtstephens.com/files/p372-steele.pdf.
template <typename Double>
@@ -916,52 +942,74 @@ FMT_FUNC void fallback_format(Double v, buffer<char>& buf, int& exp10) {
// Shift to account for unequal gaps when lower boundary is 2 times closer.
// TODO: handle denormals
int shift = fp_value.f == 1 ? 1 : 0;
// Shift value and pow10 by an extra bit to make lower and upper which are
// normally half ulp integers. This eliminates multiplication by 2 during
// later computations.
bigint value(fp_value.f << (shift + 1)); // 2 * R in (FPP)^2.
bigint pow10; // 2 * S in (FPP)^2.
bigint lower(1); // (M^- in (FPP)^2).
bigint upper(1 << shift); // (M^+ in (FPP)^2).
bigint numerator; // 2 * R in (FPP)^2.
bigint denominator; // 2 * S in (FPP)^2.
bigint lower; // (M^- in (FPP)^2).
bigint upper_store;
bigint *upper = nullptr; // (M^+ in (FPP)^2).
// Shift numerator and denominator by an extra bit to make lower and upper
// which are normally half ulp integers. This eliminates multiplication by 2
// during later computations.
uint64_t significand = fp_value.f << (shift + 1);
if (fp_value.e >= 0) {
value <<= fp_value.e;
numerator.assign(significand);
numerator <<= fp_value.e;
lower.assign(1);
lower <<= fp_value.e;
upper <<= fp_value.e;
pow10.assign_pow10(exp10);
pow10 <<= 1;
if (shift != 0) {
upper_store.assign(1);
upper_store <<= fp_value.e + shift;
upper = &upper_store;
} else {
upper = &lower;
}
denominator.assign_pow10(exp10);
denominator <<= 1;
} else {
pow10 <<= -fp_value.e;
// TODO: fixup
numerator.assign_pow10(-exp10);
lower.assign(numerator);
if (shift != 0) {
upper_store.assign(numerator);
upper_store <<= 1;
upper = &upper_store;
} else {
upper = &lower;
}
numerator *= significand;
denominator.assign(1);
denominator <<= 1 - fp_value.e;
}
// Invariant: fp_value == (value / pow10) * pow(10, exp10).
// Invariant: fp_value == (numerator / denominator) * pow(10, exp10).
bool even = (fp_value.f & 1) == 0;
int num_digits = 0;
char* data = buf.data();
for (;;) {
int digit = value.divmod_assign(pow10);
bool low = compare(value, lower) - even < 0; // value <[=] lower.
bool high = add_compare(value, upper, pow10) + even >
0; // value + upper >[=] pow10.
int digit = numerator.divmod_assign(denominator);
bool low = compare(numerator, lower) - even < 0; // numerator <[=] lower.
bool high = add_compare(numerator, *upper, denominator) + even >
0; // numerator + upper >[=] pow10.
if (low || high) {
if (!low) {
++digit;
} else if (high) {
// TODO: round up if 2 * value >= pow10
// TODO: round up if 2 * numerator >= denominator
}
data[num_digits++] = static_cast<char>('0' + digit);
buf.resize(num_digits);
exp10 -= num_digits -1;
return;
}
data[num_digits++] = static_cast<char>('0' + digit);
numerator *= 10;
lower *= 10;
upper *= 10;
if (upper != &lower) *upper *= 10;
}
}
template <typename Double,
enable_if_t<(sizeof(Double) == sizeof(uint64_t)), int>>
FMT_API bool grisu_format(Double value, buffer<char>& buf, int precision,
unsigned options, int& exp) {
bool grisu_format(Double value, buffer<char>& buf, int precision,
unsigned options, int& exp) {
FMT_ASSERT(value >= 0, "value is negative");
const bool fixed = (options & grisu_options::fixed) != 0;
if (value <= 0) { // <= instead of == to silence a warning.
@@ -1003,7 +1051,14 @@ FMT_API bool grisu_format(Double value, buffer<char>& buf, int precision,
assert(min_exp <= upper.e && upper.e <= -32);
auto result = digits::result();
int size = 0;
if ((options & grisu_options::grisu3) != 0) {
if ((options & grisu_options::grisu2) != 0) {
++lower.f; // \tilde{M}^- + 1 ulp -> M^-_{\uparrow}.
--upper.f; // \tilde{M}^+ - 1 ulp -> M^+_{\downarrow}.
grisu_shortest_handler<2> handler{buf.data(), 0, (upper - normalized).f};
result = grisu_gen_digits(upper, upper.f - lower.f, exp, handler);
size = handler.size;
assert(result != digits::error);
} else {
--lower.f; // \tilde{M}^- - 1 ulp -> M^-_{\downarrow}.
++upper.f; // \tilde{M}^+ + 1 ulp -> M^+_{\uparrow}.
// Numbers outside of (lower, upper) definitely do not round to value.
@@ -1011,17 +1066,10 @@ FMT_API bool grisu_format(Double value, buffer<char>& buf, int precision,
result = grisu_gen_digits(upper, upper.f - lower.f, exp, handler);
size = handler.size;
if (result == digits::error) {
exp -= cached_exp10;
exp = exp + size - cached_exp10 - 1;
fallback_format(value, buf, exp);
return true;
}
} else {
++lower.f; // \tilde{M}^- + 1 ulp -> M^-_{\uparrow}.
--upper.f; // \tilde{M}^+ - 1 ulp -> M^+_{\downarrow}.
grisu_shortest_handler<2> handler{buf.data(), 0, (upper - normalized).f};
result = grisu_gen_digits(upper, upper.f - lower.f, exp, handler);
size = handler.size;
assert(result != digits::error);
}
buf.resize(to_unsigned(size));
}

2
include/fmt/format.h
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@@ -1111,7 +1111,7 @@ It grisu_prettify(const char* digits, int size, int exp, It it,
}
namespace grisu_options {
enum { fixed = 1, grisu3 = 2 };
enum { fixed = 1, grisu2 = 2 };
}
// Formats value using the Grisu algorithm:

9
test/format-impl-test.cc
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@@ -87,16 +87,19 @@ TEST(BigIntTest, ShiftLeft) {
TEST(BigIntTest, Multiply) {
bigint n(0x42);
EXPECT_THROW(n *= 0, assertion_failure);
n *= 1;
EXPECT_EQ("42", fmt::format("{}", n));
n *= 2;
EXPECT_EQ("84", fmt::format("{}", n));
n *= 0x12345678;
EXPECT_EQ("962fc95e0", fmt::format("{}", n));
auto max = max_value<uint32_t>();
bigint bigmax(max);
bigmax *= max;
bigint bigmax(max_value<uint32_t>());
bigmax *= max_value<uint32_t>();
EXPECT_EQ("fffffffe00000001", fmt::format("{}", bigmax));
bigmax.assign(max_value<uint64_t>());
bigmax *= max_value<uint64_t>();
EXPECT_EQ("fffffffffffffffe0000000000000001", fmt::format("{}", bigmax));
}
TEST(BigIntTest, Accumulator) {

9
test/grisu-test.cc
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@@ -55,3 +55,12 @@ TEST(GrisuTest, Prettify) {
}
TEST(GrisuTest, ZeroPrecision) { EXPECT_EQ("1", fmt::format("{:.0}", 1.0)); }
TEST(GrisuTest, Fallback) {
EXPECT_EQ("1e+23", fmt::format("{}", 1e23));
EXPECT_EQ("9e-265", fmt::format("{}", 9e-265));
EXPECT_EQ("5.423717798060526e+125",
fmt::format("{}", 5.423717798060526e+125));
EXPECT_EQ("1.372371880954233e-288",
fmt::format("{}", 1.372371880954233e-288));
}