Joseph Ferano
57fe91f303
FIX.org, NEXT.md, DISCUSS.org, docs/DISCUSS.md and the session handoff at the root are one TODO.org now: 293 entries under seven subsystem headings, each carrying an org keyword that says where it stands. A DONE entry is a few lines saying what was decided and what that rules out; the reasoning that would not compress — the embedding spike and the four reports the hand-written x86 backend was built from — moved into docs/BUILT.md instead, and its entries point there in one line. Every entry was checked against the tree before it got a keyword, and the prose was wrong in both directions. Things the deleted files called open were built: the first-evaluation stall, main being redefinable, macro parameter lists, the type-limit constants, the array constructors, the byte fills, inc/dec, the discard's fontification, the Emacs buffers, rt_die's _exit, the backtrace surface, and the acceptance failure that could print and still exit zero. Things they called done were not: the backend reports' no-plan buckets had gone stale in the other direction, the value-dependent defvar was superseded rather than built, and macro-expansion source locations are on an unmerged lane, so that entry is NEXT and names the branch. Every comment that cited one of the five by name now cites a heading that exists, in TODO.org or in docs/BUILT.md. The session reports under docs/handoffs/ keep naming the files they worked on, because rewriting them would falsify what those sessions did; each carries a note saying where the content went.
79 lines
3.3 KiB
Plaintext
79 lines
3.3 KiB
Plaintext
;;;; atan2, pow, and clamp — three of the functions TODO.org, "The second tier
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;;;; of the standard library", names.
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;;;;
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;;;; Two of them are declares over libm and the third is a macro, and that
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;;;; split is the whole content of this file. atan2f and powf are not correctly
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;;;; rounded under IEEE-754, exactly as sinf and cosf are not, so every case
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;;;; below is a value whose answer is exact in binary — a quadrant boundary, a
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;;;; power of two, a perfect square — rather than one that would pin a
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;;;; particular libm's last bit and then fail on wasi-libc.
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;;;;
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;;;; clamp is a macro because a function could not be: min and max are builtins
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;;;; at every numeric type and there are no generics, so a clamp function is
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;;;; one copy per type. The proof that the macro is not one copy per type is
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;;;; that the same three-word call below is made at i32, i64, u8 and f32.
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(defn show [x f32] ()
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(print x)
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(print " "))
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;;; clamp must evaluate each of its three arguments exactly once. A macro that
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;;; repeated one — say (if (< x lo) lo (if (> x hi) hi x)), which names x twice
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;;; — would read identically and call this twice.
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(defonce calls i32 0)
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(defn tick [x i32] i32
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(set calls (+ calls 1))
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x)
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(defn main [] i32
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;; atan2 across all four quadrants and on both axes, which is the whole
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;; reason for it over a division: the quadrant is recovered from two signs,
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;; and (/ y x) has thrown it away before atan sees it. The two on the x = 0
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;; axis are where the division does not exist at all.
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(show (atan2-f32 1.0 1.0)) ; pi/4
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(show (atan2-f32 1.0 -1.0)) ; 3pi/4
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(show (atan2-f32 -1.0 -1.0)) ; -3pi/4
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(show (atan2-f32 -1.0 1.0)) ; -pi/4
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(show (atan2-f32 0.0 1.0)) ; 0
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(show (atan2-f32 1.0 0.0)) ; pi/2
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(show (atan2-f32 -1.0 0.0)) ; -pi/2
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(println "")
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;; pow. A power of two, a half-power that is a perfect square, a negative
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;; exponent, and the two edge exponents every implementation special-cases.
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(show (pow-f32 2.0 10.0)) ; 1024
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(show (pow-f32 9.0 0.5)) ; 3
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(show (pow-f32 2.0 -2.0)) ; 0.25
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(show (pow-f32 5.0 0.0)) ; 1
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(show (pow-f32 5.0 1.0)) ; 5
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(println "")
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;; clamp: below the range, above it, and inside it untouched.
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(print (clamp 0 1 3)) (print " ") ; 1
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(print (clamp 5 1 3)) (print " ") ; 3
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(print (clamp 2 1 3)) (print " ") ; 2
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;; Both ends are inclusive, which is the off-by-one a hand-written clamp
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;; gets wrong with a < where it wanted <=.
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(print (clamp 1 1 3)) (print " ") ; 1
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(print (clamp 3 1 3)) ; 3
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(println "")
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;; The same three words at three more types, none of which a function could
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;; have served without a copy of its own.
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(print (clamp (i64 900) (i64 0) (i64 255))) (print " ") ; 255
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(print (clamp (u8 200) (u8 0) (u8 255))) (print " ") ; 200
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(show (clamp 2.5 0.0 1.0)) ; 1
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(println "")
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;; lo above hi is not an error and answers hi: the outer min wins. Written
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;; down here rather than left for someone to discover.
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(print (clamp 7 5 1))
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(println "")
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;; Three arguments, three evaluations.
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(print (clamp (tick 5) (tick 1) (tick 3))) (print " ")
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(print calls)
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(println "")
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0)
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