// Package history keeps a fixed-size in-memory ring of metric samples.
package history

import (
	"sync"
	"time"
)

type Sample struct {
	T       int64   `json:"t"` // unix seconds
	CPU     float64 `json:"cpu"`
	MemUsed uint64  `json:"mem_used"`
	ARC     uint64  `json:"arc"`
	CPUTemp float64 `json:"cpu_temp"`
}

type Ring struct {
	mu   sync.RWMutex
	buf  []Sample
	head int
	n    int
}

func New(size int) *Ring { return &Ring{buf: make([]Sample, size)} }

func (r *Ring) Add(s Sample) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.buf[r.head] = s
	r.head = (r.head + 1) % len(r.buf)
	if r.n < len(r.buf) {
		r.n++
	}
}

// Since returns samples newer than t in chronological order, thinned to at
// most maxPoints entries.
func (r *Ring) Since(t time.Time, maxPoints int) []Sample {
	r.mu.RLock()
	defer r.mu.RUnlock()
	ts := t.Unix()
	out := make([]Sample, 0, r.n)
	start := (r.head - r.n + len(r.buf)) % len(r.buf)
	for i := 0; i < r.n; i++ {
		s := r.buf[(start+i)%len(r.buf)]
		if s.T >= ts {
			out = append(out, s)
		}
	}
	if maxPoints > 0 && len(out) > maxPoints {
		step := (len(out) + maxPoints - 1) / maxPoints
		thin := out[:0:0]
		for i := 0; i < len(out); i += step {
			thin = append(thin, out[i])
		}
		out = thin
	}
	return out
}