package buildstep

import (
	"encoding/json"
	"fmt"
	"os"
)

// CapacitySetting is the build modules' setting of the node's build capacity (novox/hq ADR 0525 §5, ADR 0567 §12): how
// many asks the node takes at once, an integer config value set on each node's layer of mesh-config. No repository names
// a node: each build module declares it with the default 0, which pulls nothing, and the controller (or mesh-config)
// lays the node's value into the module's settings file.
const CapacitySetting = "capacity"

// NodeCapacity is the node's build capacity, read from the module's settings file at path at every call, so a change
// takes effect at the next pull. No path, no file value, or a value that is not a whole number of at least 0 gives 0 and
// a value that cannot be read says why: pulling on a setting nobody can read could build on a node that must not.
func NodeCapacity(path, node string, logf func(string, ...any)) func() int {
	say := func(format string, args ...any) {
		if logf != nil {
			logf(format, args...)
		}
	}
	return func() int {
		if path == "" {
			return 0
		}
		raw, err := os.ReadFile(path)
		if err != nil {
			say("the build capacity cannot be read, so %s pulls nothing: %v", node, err)
			return 0
		}
		var s map[string]json.RawMessage
		if err := json.Unmarshal(raw, &s); err != nil {
			say("the settings file %s is not JSON, so %s pulls nothing: %v", path, node, err)
			return 0
		}
		n, err := capacityOf(s[CapacitySetting])
		if err != nil {
			say("%v, so %s pulls nothing", err, node)
			return 0
		}
		return n
	}
}

// capacityOf is the capacity a config value holds: absent or null is 0; anything but a whole number of at least 0 is
// refused, naming it.
func capacityOf(value json.RawMessage) (int, error) {
	if len(value) == 0 || string(value) == "null" {
		return 0, nil
	}
	var n int
	if err := json.Unmarshal(value, &n); err != nil || n < 0 {
		return 0, fmt.Errorf("the build capacity %s is not a whole number of asks at once, 0 or more", value)
	}
	return n, nil
}