// Package inventory answers: what is this machine, and what does it hold?
//
// Reported upward and never asked downward (novox/hq 03-DESIGN/01-to-be/05-the-node-host.md).
// Everything here is read from the machine at the moment of asking — nothing is remembered,
// nothing is derived from a file that says what the machine ought to be
// (novox/hq ADR 0018).
package inventory

import (
	"context"
	"os"
	"runtime"
	"strconv"
	"strings"
	"time"

	"github.com/novox/mesh-host/internal/profile"
)

// Inventory is what a machine reports about itself.
//
// Deliberately small. Everything here is either needed to identify the machine or needed to
// decide what may be placed on it; anything else would be a fact the mesh stores and nothing
// reads, which is the shape 04-ISSUES/003 records.
type Inventory struct {
	// Machine is what this machine calls itself. NOT its node name — a node's name is assigned
	// by the mesh, and a host that named itself would be deciding something.
	Machine string `json:"machine"`

	OS           string `json:"os"`
	Architecture string `json:"architecture"`
	Kernel       string `json:"kernel,omitempty"`
	Distribution string `json:"distribution,omitempty"`

	CPUs     int   `json:"cpus"`
	MemoryKB int64 `json:"memory_kb,omitempty"`

	Profile profile.Profile `json:"profile"`

	// ObservedAt is when this was read. An inventory with no timestamp cannot be told from a
	// stale one, and a node that has been unreachable for a week is an ordinary situation
	// (novox/hq ADR 0004) rather than an error — so the age of the observation is part of it.
	ObservedAt time.Time `json:"observed_at"`

	// Unreadable lists what could not be determined, and why. An absent field and a field that
	// failed to read are different facts, and collapsing them loses the one worth acting on.
	Unreadable []string `json:"unreadable,omitempty"`
}

// Reader supplies the machine's own files. Replaced in tests only for the parsing layer; the
// real reader is exercised against this machine as well.
type Reader func(path string) ([]byte, error)

// Collect reads the machine. It never fails: a fact that cannot be read is recorded as
// unreadable rather than aborting, because an inventory missing one field is useful and an
// inventory that refused to be taken is not.
func Collect(ctx context.Context, read Reader, detectors []profile.Detector, timeout time.Duration) Inventory {
	if read == nil {
		read = os.ReadFile
	}
	inv := Inventory{
		OS:           runtime.GOOS,
		Architecture: runtime.GOARCH,
		CPUs:         runtime.NumCPU(),
		ObservedAt:   time.Now().UTC(),
	}

	if name, err := os.Hostname(); err == nil {
		inv.Machine = name
	} else {
		inv.Unreadable = append(inv.Unreadable, "machine name: "+err.Error())
	}

	if b, err := read("/proc/sys/kernel/osrelease"); err == nil {
		inv.Kernel = strings.TrimSpace(string(b))
	} else {
		inv.Unreadable = append(inv.Unreadable, "kernel: "+err.Error())
	}

	if b, err := read("/etc/os-release"); err == nil {
		inv.Distribution = distributionFrom(string(b))
	} else {
		inv.Unreadable = append(inv.Unreadable, "distribution: "+err.Error())
	}

	if b, err := read("/proc/meminfo"); err == nil {
		if kb, ok := memoryFrom(string(b)); ok {
			inv.MemoryKB = kb
		} else {
			inv.Unreadable = append(inv.Unreadable, "memory: MemTotal not found in /proc/meminfo")
		}
	} else {
		inv.Unreadable = append(inv.Unreadable, "memory: "+err.Error())
	}

	inv.Profile = profile.Detect(ctx, detectors, timeout)
	return inv
}

// distributionFrom pulls the human name out of an os-release file.
//
// Prefers PRETTY_NAME, falls back to ID. Values may be quoted or not, and a line may contain
// an `=` in the value, so the split is on the first only.
func distributionFrom(osRelease string) string {
	fields := map[string]string{}
	for _, line := range strings.Split(osRelease, "\n") {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		key, value, found := strings.Cut(line, "=")
		if !found {
			continue
		}
		fields[strings.TrimSpace(key)] = strings.Trim(strings.TrimSpace(value), `"'`)
	}
	if pretty := fields["PRETTY_NAME"]; pretty != "" {
		return pretty
	}
	return fields["ID"]
}

// memoryFrom reads MemTotal, in kilobytes, from a meminfo file.
func memoryFrom(meminfo string) (int64, bool) {
	for _, line := range strings.Split(meminfo, "\n") {
		if !strings.HasPrefix(line, "MemTotal:") {
			continue
		}
		parts := strings.Fields(line)
		if len(parts) < 2 {
			return 0, false
		}
		kb, err := strconv.ParseInt(parts[1], 10, 64)
		if err != nil {
			return 0, false
		}
		return kb, true
	}
	return 0, false
}