1 files changed, 183 insertions, 0 deletions
diff --git a/exchange.go b/exchange.go
new file mode 100644
index 0000000..49f0a2f
--- /dev/null
+++ b/exchange.go
@@ -0,0 +1,183 @@
+// TODO
+package exchange
+
+import (
+	"bytes"
+	"cmp"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"slices"
+
+	"github.com/google/go-querystring/query"
+	"github.com/jellydator/ttlcache/v3"
+	"github.com/shopspring/decimal"
+)
+
+const (
+	keyHdr              = "X-CMC_PRO_API_KEY"
+	priceConversionPath = "/v2/tools/price-conversion"
+)
+
+var apiUrl = mustParseUrl("https://pro-api.coinmarketcap.com")
+
+// pair is a pair of currency codes.
+type pair struct {
+	from, to string
+}
+
+type rateResult struct {
+	rate decimal.Decimal
+	err  error
+}
+
+type Client struct {
+	c      *http.Client
+	apiKey string
+	rates  *ttlcache.Cache[pair, rateResult]
+}
+
+func NewClient(apiKey string, o ...Option) *Client {
+	opts := parseOpts(o)
+
+	c := &Client{
+		&http.Client{},
+		apiKey,
+		nil,
+	}
+
+	var rateLoader ttlcache.LoaderFunc[pair, rateResult] = c.loadRate
+	rates := ttlcache.New[pair, rateResult](
+		ttlcache.WithTTL[pair, rateResult](opts.ttl),
+		ttlcache.WithLoader[pair, rateResult](rateLoader))
+	go rates.Start()
+	c.rates = rates
+
+	return c
+}
+
+func (c *Client) Close() {
+	c.c.CloseIdleConnections()
+	c.rates.Stop()
+	c.rates.DeleteAll()
+}
+
+// Rate takes two 3-letter currency codes and returns the conversion rate
+// between them such that 1×from = rate×to.
+func (c *Client) Rate(from, to string) (decimal.Decimal, error) {
+	item := c.rates.Get(pair{from, to})
+	if item.IsExpired() {
+		lg.Printf("warning: cache[{%s, %s}] is expired\n", from, to)
+	}
+	res := item.Value()
+	return res.rate, res.err
+}
+
+// loadRate retrieves an exchange rate and stores it in the cache.
+func (c *Client) loadRate(cache *ttlcache.Cache[pair, rateResult], key pair) *ttlcache.Item[pair, rateResult] {
+	from, to := key.from, key.to
+	debugf("load rate %s->%s\n", from, to)
+	query, err := query.Values(priceConversionQuery{1, from, to})
+	if err != nil {
+		return rateErr(cache, key, err)
+	}
+
+	var convs []priceConversion
+	if err := get(c, priceConversionPath, query, &convs); err != nil {
+		return rateErr(cache, key, err)
+	}
+
+	slices.SortFunc(convs, func(a, b priceConversion) int { return cmp.Compare(a.Id, b.Id) })
+	if len(convs) < 1 {
+		return rateErr(cache, key, errBadCurrency{to})
+	}
+	conv := convs[0]
+	debugf("%s: using ID %d\n", to, conv.Id)
+
+	rate, ok := conv.Quote[to]
+	if !ok {
+		return rateErr(cache, key, errBadCurrency{to})
+	}
+	if rate.Price.IsZero() {
+		lg.Printf("warning: exchange rate %s->%s is zero\n", from, to)
+	}
+	debugf("cache rate %s->%s: %v\n", from, to, rate.Price)
+	return cache.Set(key, rateResult{rate.Price, nil}, ttlcache.DefaultTTL)
+}
+
+func get[T any](c *Client, path string, query url.Values, resp *T) error {
+	// Construct request
+	uri := apiUrl.JoinPath(path).String() + "?" + query.Encode()
+	debugf("GET %v\n", uri)
+	req, err := http.NewRequest(http.MethodGet, uri, nil)
+	if err != nil {
+		return err
+	}
+	req.Header.Set(keyHdr, c.apiKey)
+
+	// Send request
+	hresp, err := c.c.Do(req)
+	if err != nil {
+		return err
+	}
+	defer hresp.Body.Close()
+
+	// Decode response
+	if hresp.StatusCode == http.StatusOK {
+		var err error
+		*resp, err = decOkResp[T](hresp)
+		return err
+	} else {
+		return decErrResp(hresp)
+	}
+}
+
+func decOkResp[T any](hresp *http.Response) (T, error) {
+	var resp response[T]
+
+	// TODO: remove once debugged
+	buf := new(bytes.Buffer)
+	_, err := buf.ReadFrom(hresp.Body)
+	if err != nil {
+		return resp.Data, err
+	}
+	debugf("raw response: %s\n", buf.String())
+	dec := json.NewDecoder(buf)
+
+	err = dec.Decode(&resp)
+	debugf("decoded response: %v\n", resp)
+	if err == nil && resp.Status.Error != "" {
+		err = fmt.Errorf("%s", resp.Status.Error)
+	}
+	return resp.Data, err
+}
+
+func decErrResp(hresp *http.Response) error {
+	var errInfo apiError
+	dec := json.NewDecoder(hresp.Body)
+	if err := dec.Decode(&errInfo); err != nil {
+		return err
+	}
+	debugf("%v\n", errInfo)
+	if err := errInfo.Status.Error; err != "" {
+		return fmt.Errorf("%s", err)
+	} else {
+		return fmt.Errorf("CoinMarketCap error: %s", hresp.Status)
+	}
+}
+
+func mustParseUrl(s string) *url.URL {
+	u, err := url.Parse(s)
+	if err != nil {
+		panic(err)
+	}
+	return u
+}
+
+func rateErr(cache *ttlcache.Cache[pair, rateResult], key pair, err error) *ttlcache.Item[pair, rateResult] {
+	debugf("rate error: %v: %v\n", key, err)
+	var res rateResult
+	res.err = err
+	return cache.Set(key, res, ttlcache.DefaultTTL)
+}