// Package buildcpu is how much of a build node's processors its builds may take, and how they yield to everything else
// there (novox/hq issue 1806; the operator, 2026-10-26: "limit CPU usage on our build nodes so the total cpu usage never
// impacts the normal workings of our nodes").
//
// Three parts, each with one owner:
//
//   - **Priority.** A build seat's holder (catalogue.Seat.Yields) runs at the lowest CPU and IO priority there is: the
//     composer gives its runtime process declaration.Yields, and the node-engine writes it into the unit as
//     CPUWeight=idle, IOWeight=1, Nice=19 and the best-effort IO class at its lowest level. Interactive and service
//     work always comes first; builds take the cycles nothing else wants.
//   - **The cap.** The build modules' setting `cpu-cap` (Setting) gives each node the share of its processors its builds
//     may take at most, as `<node>=<percent>` pairs. The composer turns this node's share into Yields.CPUShare, and the
//     node-engine into CPUQuota for the machine's processor count: a hard bound, held by the kernel.
//   - **Threads.** Inside that bound a step starts no more threads than its part of it (Threads): GOMAXPROCS, `go
//     test -p`, the Node test runner's concurrency. A step with more runnable threads than its quota only waits in the
//     kernel's throttle, and its timeouts with it.
package buildcpu

import (
	"fmt"
	"os"
	"runtime"
	"sort"
	"strconv"
	"strings"
)

// Setting is the build modules' setting of each node's processor cap: `<node>=<percent>` pairs separated by commas,
// such as `shanks=50,ace=75`; a percent sign after the number is allowed. Each build module declares it with Defaults
// as its default.
const Setting = "cpu-cap"

// Defaults is the setting's default: the operator's two desktops (shanks, g14) keep half their processors for
// themselves at all times, and ace, a server, builds on three quarters. A node it does not name has DefaultPercent.
const Defaults = "shanks=50,g14=50,ace=75"

// DefaultPercent is the cap of a node the setting does not name.
const DefaultPercent = 50

// Parse reads the setting's pairs. A percent outside 1 to 100 is refused: 0 would stop every build without pausing the
// seat (the capacity setting says that, with 0 asks), and more than 100 is no cap.
func Parse(s string) (map[string]int, error) {
	out := map[string]int{}
	for _, pair := range strings.Split(s, ",") {
		pair = strings.TrimSpace(pair)
		if pair == "" {
			continue
		}
		node, value, ok := strings.Cut(pair, "=")
		node = strings.TrimSpace(node)
		value = strings.TrimSuffix(strings.TrimSpace(value), "%")
		n, err := strconv.Atoi(strings.TrimSpace(value))
		if !ok || node == "" || err != nil || n < 1 || n > 100 {
			return nil, fmt.Errorf("the processor cap %q is not <node>=<percent of its processors, 1 to 100>", pair)
		}
		if _, dup := out[node]; dup {
			return nil, fmt.Errorf("the processor cap names %s twice", node)
		}
		out[node] = n
	}
	return out, nil
}

// Of is node's cap in percent of its processors: DefaultPercent for a node the setting does not name.
func Of(setting, node string) (int, error) {
	caps, err := Parse(setting)
	if err != nil {
		return 0, err
	}
	if n, ok := caps[node]; ok {
		return n, nil
	}
	return DefaultPercent, nil
}

// Format is a cap map in the setting's own words, sorted by node.
func Format(caps map[string]int) string {
	nodes := make([]string, 0, len(caps))
	for n := range caps {
		nodes = append(nodes, n)
	}
	sort.Strings(nodes)
	parts := make([]string, 0, len(nodes))
	for _, n := range nodes {
		parts = append(parts, fmt.Sprintf("%s=%d", n, caps[n]))
	}
	return strings.Join(parts, ",")
}

// Quota is the unit's CPUQuota for a share of a machine with cores processors: systemd counts a quota in percent of
// one processor, so half of 24 is 1200%. At least 100%, one processor, so a cap on a small machine never stops a step.
func Quota(percent, cores int) int {
	q := percent * cores
	if q < 100 {
		q = 100
	}
	return q
}

// Threads is how many threads one step may run at once: its part of the cap when slots steps run beside it, at least
// one. Rounded down, so the steps together stay inside the quota.
func Threads(percent, cores, slots int) int {
	if slots < 1 {
		slots = 1
	}
	if percent < 1 || percent > 100 {
		percent = 100
	}
	t := cores * percent / 100 / slots
	if t < 1 {
		t = 1
	}
	return t
}

// Limits is what one step is held to on this node, as a status tool says it.
type Limits struct {
	// Percent is the node's cap in percent of its processors, and Setting the words it came from.
	Percent int    `json:"cap-percent"`
	Setting string `json:"cap-setting"`
	// Cores is the machine's processor count.
	Cores int `json:"cores"`
	// Slots is the node's build capacity: steps at once.
	Slots int `json:"slots"`
	// Threads is what each step is given (Threads).
	Threads int `json:"threads-per-step"`
}

// LimitsFor is the limits of a step on node, from the cap setting and the node's capacity. A setting that cannot be read
// gives the default cap and says why in the error, so a step still runs with a bound rather than none.
func LimitsFor(setting, node string, slots int) (Limits, error) {
	l := Limits{Setting: setting, Cores: runtime.NumCPU(), Slots: slots}
	p, err := Of(setting, node)
	if err != nil {
		p = DefaultPercent
	}
	l.Percent = p
	l.Threads = Threads(p, l.Cores, slots)
	return l, err
}

// Env is the environment a step's commands are given so they start no more threads than they may: GOMAXPROCS for every
// Go program (the compiler, a test binary, esbuild, buildah), UV_THREADPOOL_SIZE for Node's worker pool. Returned as
// KEY=value words to append to a command's environment.
func Env(threads int) []string {
	if threads < 1 {
		threads = 1
	}
	t := strconv.Itoa(threads)
	return []string{"GOMAXPROCS=" + t, "UV_THREADPOOL_SIZE=" + t}
}

// WithEnv is env with Env(threads) in place of any GOMAXPROCS or UV_THREADPOOL_SIZE it carried.
func WithEnv(env []string, threads int) []string {
	out := make([]string, 0, len(env)+2)
	for _, kv := range env {
		if strings.HasPrefix(kv, "GOMAXPROCS=") || strings.HasPrefix(kv, "UV_THREADPOOL_SIZE=") {
			continue
		}
		out = append(out, kv)
	}
	return append(out, Env(threads)...)
}

// InForce is what the kernel holds this process to, read rather than assumed (evidence over assertion): its nice
// level, its cgroup, the cgroup's CPU weight, idle mark and quota. A field the machine does not answer is left empty
// and Unread says which.
type InForce struct {
	Nice      int    `json:"nice"`
	Cgroup    string `json:"cgroup,omitempty"`
	CPUIdle   string `json:"cpu-idle,omitempty"`
	CPUWeight string `json:"cpu-weight,omitempty"`
	// CPUMax is the cgroup's cpu.max: "<quota µs> <period µs>", or "max" with no quota.
	CPUMax string `json:"cpu-max,omitempty"`
	// Quota is CPUMax as a percent of one processor, 0 with none.
	Quota    int      `json:"quota-percent,omitempty"`
	IOWeight string   `json:"io-weight,omitempty"`
	Unread   []string `json:"unread,omitempty"`
}

// cgroupRoot is where the unified hierarchy is mounted. A variable so a test can point it elsewhere.
var cgroupRoot = "/sys/fs/cgroup"

// procSelfCgroup is this process's cgroup file. A variable so a test can point it elsewhere.
var procSelfCgroup = "/proc/self/cgroup"

// ReadInForce reads what the kernel holds this process to.
func ReadInForce() InForce {
	var f InForce
	f.Nice = nice()
	raw, err := os.ReadFile(procSelfCgroup)
	if err != nil {
		f.Unread = append(f.Unread, "cgroup: "+err.Error())
		return f
	}
	for _, line := range strings.Split(strings.TrimSpace(string(raw)), "\n") {
		if strings.HasPrefix(line, "0::") {
			f.Cgroup = strings.TrimPrefix(line, "0::")
		}
	}
	if f.Cgroup == "" {
		f.Unread = append(f.Unread, "cgroup: not on the unified hierarchy")
		return f
	}
	dir := cgroupRoot + f.Cgroup
	read := func(name string) string {
		b, err := os.ReadFile(dir + "/" + name)
		if err != nil {
			f.Unread = append(f.Unread, name+": "+err.Error())
			return ""
		}
		return strings.TrimSpace(string(b))
	}
	f.CPUIdle = read("cpu.idle")
	f.CPUWeight = read("cpu.weight")
	f.CPUMax = read("cpu.max")
	f.IOWeight = read("io.weight")
	f.Quota = quotaOf(f.CPUMax)
	return f
}

// quotaOf is a cpu.max value as a percent of one processor: "1200000 100000" is 1200; "max ..." or nothing is 0.
func quotaOf(cpuMax string) int {
	fields := strings.Fields(cpuMax)
	if len(fields) != 2 || fields[0] == "max" {
		return 0
	}
	q, err1 := strconv.Atoi(fields[0])
	p, err2 := strconv.Atoi(fields[1])
	if err1 != nil || err2 != nil || p <= 0 {
		return 0
	}
	return q * 100 / p
}

// procSelfStat is this process's stat file. A variable so a test can point it elsewhere.
var procSelfStat = "/proc/self/stat"

// nice is this process's nice level from its stat file (the 19th field, counted after the command's closing
// parenthesis, which may itself hold spaces); 0 when it cannot be read.
func nice() int {
	raw, err := os.ReadFile(procSelfStat)
	if err != nil {
		return 0
	}
	s := string(raw)
	i := strings.LastIndex(s, ")")
	if i < 0 {
		return 0
	}
	fields := strings.Fields(s[i+1:])
	// After the command: state is field 3, so nice (field 19) is index 16.
	if len(fields) < 17 {
		return 0
	}
	n, _ := strconv.Atoi(fields[16])
	return n
}