Joseph Ferano
9ce51ba94e
as-slice was a warning, not an operation. The input type already decides which of the two things happens — a Vec can only be borrowed, an array or a string can only be viewed, and no call site picks between them — so the second name expressed no choice a reader could make. And it warned at the moment the view is taken, which is the one moment nothing is wrong; the danger arrives later, at the push. slice now takes a Vec at all three arities and as-slice is gone. (slice v lo) was free, and is the arity the Vec never had: the runtime already reads a hi of -1 as "to the end", so the tail form passes the caller's lo and the same -1 — no slot, no length read, no second evaluation. The merge is entirely in the checker; the Vec path builds the flan_vec_as_slice call it always built and neither backend has a line about any of it. A Vec a call returned is refused at every arity, and not for the array's reason. (slice (mk)) over an array dangles. (slice (make-vec)) does not — the storage outlives the expression — but the header is a temporary, so nothing can ever free the block. The refusal says that and names the let. The name's own refusal sits in ordinary_call after every table, so a program that defines an as-slice still reaches its own. It reads for somebody who has never heard of the old name and writes the call back out, spelling each argument that is a name or a number. The warning moved to where it bites: BUILT.md gains a section beside the Vec table and the push row points at it, spec-memory.md's Borrowing says the same. Investigated and deliberately not built — a diagnostic for a live view at the push. (reserve v 100) then a slice, a push and a read is correct code under the contract the spec chose, so any flag on it is a false positive by the language's own semantics rather than by an approximation. FIX.org has the finding and the syntactic sketch that does not work.
101 lines
4.6 KiB
Plaintext
101 lines
4.6 KiB
Plaintext
;;;; format-f64: a number rendered to a fixed number of decimal places.
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;;;;
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;;;; The runtime's f64->bytes is snprintf "%g" and there is no precision to
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;;;; pass it, so this is the first number formatter in the language that a
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;;;; caller can steer. Every case below is one a plausible wrong version gets
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;;;; wrong, and three of them are the ones that actually ship broken: the
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;;;; carry, where the rounded fraction equals the scale and is not a fraction
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;;;; at all; the zero padding, without which 1.005 prints as "1.5"; and the
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;;;; sign, which belongs to the number and not to its integer part, because
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;;;; -0.5 has an integer part of 0 and 0 carries no sign.
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(defn show [x f64 p i32] ()
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(let [v (format-f64 x p)]
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(println (string (slice v)))
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(free v)))
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(defn main [] i32
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;; The ordinary cases, and the one %g cannot do at all: 1/60 wanted to two
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;; places is a frame time, and "%g" answers 0.0166667.
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(show 3.14159 2) ; 3.14
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(show 0.0166667 2) ; 0.02
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(show 1234.5 1) ; 1234.5
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(show 2.0 0) ; 2
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(show 2.0 3) ; 2.000
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;; Rounding is half away from zero at the last digit kept, on both signs.
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(show 0.125 2) ; 0.13
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(show -0.125 2) ; -0.13
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(show 2.5 0) ; 3
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(show -2.5 0) ; -3
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;; The carry. 0.999995 scaled by 10^5 rounds to exactly 100000, which is the
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;; next integer; without the carry this prints "0.100000".
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(show 0.999995 5) ; 1.00000
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(show 9.99 1) ; 10.0
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(show -9.99 1) ; -10.0
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(show 0.99 0) ; 1
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;; Zero padding. The fraction of 1.005 at three places is 5, and five digits
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;; is not the same number as 005.
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(show 1.005 3) ; 1.005
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(show 1.0001 4) ; 1.0001
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(show 7.0 6) ; 7.000000
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;; The sign lives on the number, not on the integer part: both of these have
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;; an integer part of 0, which i64->bytes renders without a sign.
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(show -0.5 2) ; -0.50
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(show -0.004 2) ; -0.00
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;; -0.0 prints as a plain zero. The sign test is (< x 0.0), which -0.0 fails,
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;; and text is not where the sign of a zero should be read from.
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(show 0.0 2) ; 0.00
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(show -0.0 2) ; 0.00
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;; Precision is clamped rather than refused, at both ends.
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(show 1.5 -3) ; 2
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(show 1.5 40) ; 1.500000000
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;; The three inputs with no decimal expansion.
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(show (/ 0.0 0.0) 2) ; nan
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(show (/ 1.0 0.0) 2) ; inf
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(show (/ -1.0 0.0) 2) ; -inf
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;; The same three through print, which goes to the runtime's %g rather than
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;; to format-f64, and the first of them is here for a reason the other two
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;; are not. A NaN carries a sign bit that no arithmetic chose: LLVM folds
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;; (/ 0.0 0.0) at compile time and answers the positive one, divsd answers
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;; the negative one at run time, and "%g" prints the difference. So this
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;; line printed "nan" through one backend and "-nan" through the other for
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;; the same source, and disagreed with the (show ...) above it inside either
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;; one. flan_f64_to_bytes now renders any NaN unsigned, which is what
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;; format-f64 always did. An infinity still prints signed: there the sign is
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;; the value, and both backends were always agreed about it.
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(print (/ 0.0 0.0)) (println "") ; nan
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(print (/ 1.0 0.0)) (println "") ; inf
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(print (/ -1.0 0.0)) (println "") ; -inf
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;; Past 9e18 an f64 has no fractional bits and the integer part does not fit
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;; in an i64, so this falls back to %g rather than approximating.
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(show 1e20 2) ; 1e+20
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;; A large magnitude that does fit, where the fraction is genuinely gone: an
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;; f64 has no bits below 1 up there, so the padding produces the zeros.
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(show 1234567890123.0 2) ; 1234567890123.00
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;; And the thing it is for: a formatted number inside a built string, which
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;; needs the integer part copied out before the fraction is rendered, because
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;; both come through the runtime's one shared scratch buffer.
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(let [b (vec-new u8)]
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(append (addr b) (bytes-view "fps "))
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(let [f (format-f64 59.94 1)]
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(append (addr b) (slice f))
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(free f))
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(append (addr b) (bytes-view " / frame "))
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(let [f (format-f64 0.0166667 4)]
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(append (addr b) (slice f))
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(free f))
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(println (string (slice b))) ; fps 59.9 / frame 0.0167
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(free b))
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0)
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