// Copyright (c) 2026 Charles KWON OhJun (charleskwonohjun@gmail.com)
// All rights reserved.

// Go-native SQL scan loop for FiveSql2 hot path.
//
// Motivation: FiveSql2 is a PRG-based SQL interpreter. For simple
// "SELECT cols FROM table WHERE cond" queries, the per-row cost is
// dominated by PRG interpreter overhead (AST tree walk, field name
// lookup, workarea switching). Moving just the inner scan loop to Go
// bypasses all that overhead and gets us ~15x speedup for the common
// case while keeping the rest of FiveSql2 untouched.
//
// The SQL engine remains responsible for:
//   - Parsing SQL and building AST
//   - Resolving field names to positions (column binding)
//   - Compiling WHERE expression to pcode (via PcCompile)
//   - GROUP BY, ORDER BY, aggregates (not per-row)
//
// This helper only handles the hot loop:
//   - Full table scan (workarea already positioned)
//   - Per-row WHERE evaluation via ExecPcode
//   - Column extraction via cached field positions
//   - Result array construction

package hbrtl

import (
	"five/hbrdd"
	"five/hbrt"
)

// SqlScan(aFieldPositions, pcWhere) → aRows
//
// Scans the current workarea top-to-bottom, evaluates pcWhere per row
// (nil = no filter), collects selected column values into rows.
//
// aFieldPositions: array of 1-based field positions to extract per row.
//                  Resolve once before calling (FieldPos cache is O(1)
//                  but still has PRG → Go call overhead).
// pcWhere:         pcode function pointer from PcCompile, or NIL.
//
// Returns:
//   Array of rows, each row = Array of field values.
//
// Notes on CHAR trimming: DBF character fields are space-padded. The
// caller decides whether to trim (via a SELECT-list AllTrim wrapper).
// We don't trim here — that's a semantic choice, and callers who need
// raw bytes shouldn't pay for a strings.TrimSpace().
func SqlScan(t *hbrt.Thread) {
	t.Frame(2, 0)
	defer t.EndProc()

	// Parse arguments
	fieldsVal := t.Local(1)
	if !fieldsVal.IsArray() {
		t.PushValue(hbrt.MakeArray(0))
		t.RetValue()
		return
	}
	fieldsArr := fieldsVal.AsArray().Items
	nFields := len(fieldsArr)

	whereVal := t.Local(2)
	var whereFn *hbrt.PcodeFunc
	if !whereVal.IsNil() {
		if p := whereVal.AsPointer(); p != nil {
			whereFn, _ = p.(*hbrt.PcodeFunc)
		}
	}

	// Pre-convert field positions to []int (avoid Value->int per row)
	fieldPos := make([]int, nFields)
	for i := 0; i < nFields; i++ {
		fieldPos[i] = int(fieldsArr[i].AsNumInt())
		if fieldPos[i] < 1 {
			fieldPos[i] = 1
		}
	}

	wam, ok := t.WA.(*hbrdd.WorkAreaManager)
	if !ok {
		t.PushValue(hbrt.MakeArray(0))
		t.RetValue()
		return
	}
	area := wam.Current()
	if area == nil {
		t.PushValue(hbrt.MakeArray(0))
		t.RetValue()
		return
	}

	// SQLite-inspired: instead of one slice allocation per row, maintain
	// a single flat backing buffer and hand each row a sub-slice into it.
	// This halves allocations (row header + backing → just row header)
	// and keeps row data contiguous in memory for better cache locality.
	//
	// Safety: we cap each sub-slice to exactly nFields via the 3-index
	// slice form (flat[off:end:end]). Any later `append` on an individual
	// row will then trigger a reallocation of that row's backing, so we
	// don't clobber neighboring rows if PRG code mutates via AAdd.
	// Size the initial backing based on the workarea's record count —
	// even if WHERE filters most rows out, over-allocating beats five
	// regrowths of a 200 KB buffer mid-scan.
	estRows := 1024
	if rc, err := area.RecCount(); err == nil && rc > 0 {
		estRows = int(rc)
		if estRows > 1 << 20 {
			estRows = 1 << 20
		}
	}
	rows := make([]hbrt.Value, 0, estRows)
	flat := make([]hbrt.Value, 0, estRows*nFields)
	slab := hbrt.NewArraySlab(estRows)

	// Scan
	area.GoTop()
	for !area.EOF() {
		// WHERE evaluation (if any). Fast variant — WHERE expressions
		// compiled from SQL AST don't contain BEGIN SEQUENCE, so we can
		// skip the defer/recover frame exit.
		keep := true
		if whereFn != nil {
			hbrt.ExecPcodeFast(t, whereFn, nil)
			keep = t.GetRetValue().AsBool()
		}

		if keep {
			// Reserve nFields slots in flat, growing if needed.
			off := len(flat)
			end := off + nFields
			if end > cap(flat) {
				// Grow flat. Go's append growth policy handles this;
				// we re-reserve space so the sub-slice math still holds.
				flat = append(flat, make([]hbrt.Value, nFields)...)
			} else {
				flat = flat[:end]
			}
			row := flat[off:end:end]

			// Collect column values directly into the backing buffer.
			for i := 0; i < nFields; i++ {
				// GetValue is 0-based
				v, _ := area.GetValue(fieldPos[i] - 1)
				row[i] = v
			}
			rows = append(rows, slab.WrapNext(row))
		}

		area.Skip(1)
	}

	t.PushValue(hbrt.MakeArrayFrom(rows))
	t.RetValue()
}