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Add optimized (smaller) lookup table for float type parsing #103

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d65313a
use float lookup table if parsing as float. Add two new example test …
jrahlf Sep 12, 2021
fe2be46
add missing typename for older c++ versions/compiler, rename type to …
jrahlf Sep 12, 2021
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add missing typename for older c++ versions/compiler, rename type to … 
…parsing_type
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@jrahlf
jrahlf committed Sep 12, 2021
commit fe2be46deda95e63c4ec0fd9b58b32f663ac6f65
8 changes: 4 additions & 4 deletions include/fast_float/decimal_to_binary.h
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Expand Up @@ -19,19 +19,19 @@ namespace fast_float {
template <int bit_precision>
CXX20_CONSTEXPR fastfloat_really_inline
value128 compute_product_approximation(int64_t q, uint64_t w) {
using type = std::conditional<bit_precision <= 26, float, double>::type;
const int index = 2 * int(q - powers_of_five_count<type>::smallest_power_of_five);
using parsing_type = typename std::conditional<bit_precision <= 26, float, double>::type;
const int index = 2 * int(q - powers_of_five_count<parsing_type>::smallest_power_of_five);
// For small values of q, e.g., q in [0,27], the answer is always exact because
// The line value128 firstproduct = full_multiplication(w, power_of_five_128[index]);
// gives the exact answer.
value128 firstproduct = full_multiplication(w, powers<type>::power_of_five_128[index]);
value128 firstproduct = full_multiplication(w, powers<parsing_type>::power_of_five_128[index]);
static_assert((bit_precision >= 0) && (bit_precision <= 64), " precision should be in (0,64]");
constexpr uint64_t precision_mask = (bit_precision < 64) ?
(uint64_t(0xFFFFFFFFFFFFFFFF) >> bit_precision)
: uint64_t(0xFFFFFFFFFFFFFFFF);
if((firstproduct.high & precision_mask) == precision_mask) { // could further guard with (lower + w < lower)
// regarding the second product, we only need secondproduct.high, but our expectation is that the compiler will optimize this extra work away if needed.
value128 secondproduct = full_multiplication(w, powers<type>::power_of_five_128[index + 1]);
value128 secondproduct = full_multiplication(w, powers<parsing_type>::power_of_five_128[index + 1]);
firstproduct.low += secondproduct.high;
if(secondproduct.high > firstproduct.low) {
firstproduct.high++;
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