; Latency-distribution bench fixture — Implicit-mode variant.
;
; Companion to bench_latency_explicit. Together they test the
; hypothesis "Boehm has unbounded p99 per-operation latency under
; continuous alloc pressure with a large persistent live working
; set; RC under explicit-mode has p99 within a small constant
; factor of the median".
;
; Implicit-mode variant: no `(borrow T)`, `(own T)`, `(reuse-as)`,
; `(drop-iterative)` annotations. This is the canonical "Boehm-fair"
; arm — the way you'd write the program without thinking about
; modes. Under `--alloc=gc`, Boehm cleans up. Under `--alloc=rc`,
; this variant LEAKS (Implicit params are not dec'd) and is not a
; meaningful RC measurement; the bench harness intentionally only
; runs this fixture under `--alloc=gc`.
;
; Workload:
;   - Live cache: balanced binary tree of depth 19 (524_287 nodes,
;     ~16 MB). Stays referenced through the entire bench loop.
;   - Per-op work: build a 500-cell IntList of 0..499, sum it
;     (sum = 124750), print one stdout marker line every PRINT_K
;     ops. Total churn: 20000 * 500 cells = 10M cell-allocs ≈
;     240 MB ≫ live-set, forcing Boehm to collect many times.
;   - Total ops: 20_000. Print every PRINT_K=20 ops → 1000 timing
;     samples + 1 final summary line.
;
; What the harness sees:
;   - One "READY" line at startup once the tree is built.
;   - 1000 lines, each containing the per-chunk sum (always 124750)
;     so output stays validatable. The harness ignores values and
;     records only inter-arrival times.
;   - One final "DONE" line.
;
; The harness times each line's arrival via clock_gettime on its
; end of a PTY-controlled stdout (PTY forces line-buffering through
; libc's printf), then computes median / p99 / p99.9 / max of the
; gaps.
;
; Why a print-driven gap measurement: AILang has no high-resolution
; clock extern. Adding one would mean a codegen change (a new `do
; bench/clock` op routed into the codegen seam), which is
; implementer territory, not bencher territory. Stdout-gap timing
; has a noise floor of ~10-50 µs (printf + pipe roundtrip) which is
; well below the millisecond-scale STW pauses the hypothesis
; predicts; if the hypothesis is right, the signal swamps the
; noise. If the data shows a tighter distribution than that noise
; floor, we'll have to escalate to in-process clocks; otherwise the
; bench is sufficient.

(module bench_latency_implicit

  (data Tree
    (doc "Balanced binary tree, 32-byte cells (tag + Int payload + 2 ptrs).")
    (ctor TLeaf)
    (ctor TNode (con Int) (con Tree) (con Tree)))

  (data IntList
    (doc "Singly-linked Int list, 24-byte cells.")
    (ctor LNil)
    (ctor LCons (con Int) (con IntList)))

  ; ---------- Live cache: balanced tree of given depth ----------

  (fn build_tree
    (doc "Build a balanced tree of given depth, every value = 1. Constructor-blocked — recursion depth = `depth`, fits 8MB stack at depth 19.")
    (type
      (fn-type
        (params (con Int))
        (ret (con Tree))))
    (params depth)
    (body
      (if (app == depth 0)
        (term-ctor Tree TLeaf)
        (term-ctor Tree TNode
          1
          (app build_tree (app - depth 1))
          (app build_tree (app - depth 1))))))

  (fn sum_tree
    (doc "Touch every node of the tree (ensures liveness across the loop).")
    (type
      (fn-type
        (params (con Tree))
        (ret (con Int))))
    (params t)
    (body
      (match t
        (case (pat-ctor TLeaf) 0)
        (case (pat-ctor TNode v l r)
          (app + v (app + (app sum_tree l) (app sum_tree r)))))))

  ; ---------- Per-op work: build/sum an N-cell list ----------

  (fn cons_n_acc
    (doc "Tail-recursive list builder. Result = [n-1, n-2, ..., 0] :: IntList.")
    (type
      (fn-type
        (params (con Int) (con IntList))
        (ret (con IntList))))
    (params n acc)
    (body
      (if (app == n 0)
        acc
        (tail-app cons_n_acc
          (app - n 1)
          (term-ctor IntList LCons (app - n 1) acc)))))

  (fn cons_n
    (doc "Build [0,1,...,n-1] :: IntList.")
    (type
      (fn-type
        (params (con Int))
        (ret (con IntList))))
    (params n)
    (body
      (app cons_n_acc n (term-ctor IntList LNil))))

  (fn sum_list_acc
    (doc "Tail-recursive sum.")
    (type
      (fn-type
        (params (con IntList) (con Int))
        (ret (con Int))))
    (params xs acc)
    (body
      (match xs
        (case (pat-ctor LNil) acc)
        (case (pat-ctor LCons h t)
          (tail-app sum_list_acc t (app + acc h))))))

  (fn sum_list
    (doc "Sum every element. Calls sum_list_acc with seed 0.")
    (type
      (fn-type
        (params (con IntList))
        (ret (con Int))))
    (params xs)
    (body
      (app sum_list_acc xs 0)))

  ; One operation: build and sum a list of length CHUNK_LEN, return
  ; the sum. The tree `t` is passed through and subjected to
  ; `sum_tree` so the optimizer can't eliminate it, but the result
  ; is XOR'd back into the int we return so the value chain stays
  ; live without unbounded accumulation.
  ;
  ; Note: we don't actually want sum_tree to fire on every op (it
  ; would dominate the per-op cost and bury allocator effects).
  ; Instead we touch only the tree's root via a cheap `pin_root`
  ; that pattern-matches once. Boehm's tracing still walks the
  ; whole tree on every collection because the tree pointer is
  ; live through the loop scope.
  (fn pin_root
    (doc "Constant-time tree liveness pin — read root tag, return 1 (TNode) or 0 (TLeaf).")
    (type
      (fn-type
        (params (con Tree))
        (ret (con Int))))
    (params t)
    (body
      (match t
        (case (pat-ctor TLeaf) 0)
        (case (pat-ctor TNode v l r) 1))))

  (fn one_op
    (doc "One bench operation: build+sum a fresh CHUNK_LEN-cell list, pin the tree's root, return their sum so the value chain stays observable.")
    (type
      (fn-type
        (params (con Int) (con Tree))
        (ret (con Int))))
    (params chunk_len t)
    (body
      (app + (app sum_list (app cons_n chunk_len)) (app pin_root t))))

  ; ---------- Bench loop ----------

  ; Loop runs `remaining` ops. Every PRINT_K ops, prints the
  ; rolling sum from the most-recent op (always equal to
  ; CHUNK_LEN*(CHUNK_LEN-1)/2 + 1 = 124750 + 1 = 124751 for
  ; CHUNK_LEN=500). The print is the timing event. The
  ; print_every counter's role is to keep stdout lines per second
  ; tractable for the harness (1000 timings instead of 20000).
  ;
  ; The tree `t` is passed through every recursive call so it
  ; stays a live root; Boehm has to trace through it on every
  ; collection.

  (fn loop
    (doc "Tail-recursive bench loop. Ops countdown in `remaining`; print marker every time `print_countdown` hits 0.")
    (type
      (fn-type
        (params (con Int) (con Int) (con Int) (con Int) (con Tree))
        (ret (con Unit))
        (effects IO)))
    (params remaining print_countdown chunk_len print_k t)
    (body
      (if (app == remaining 0)
        (do io/print_int 9999)
        (if (app == print_countdown 0)
          (seq
            (do io/print_int (app one_op chunk_len t))
            (tail-app loop
              (app - remaining 1)
              (app - print_k 1)
              chunk_len
              print_k
              t))
          (let _v (app one_op chunk_len t)
            (tail-app loop
              (app - remaining 1)
              (app - print_countdown 1)
              chunk_len
              print_k
              t))))))

  (fn main
    (doc "Top-level: build tree, signal READY (8888), run loop, signal DONE (9999 emitted by loop).")
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body
      (let t (app build_tree 19)
        (let _root (app pin_root t)
          (seq
            (do io/print_int 8888)
            (app loop 20000 0 500 20 t)))))))
