add: file parser mega project

x-tba/6-methods/xxx-project-file-parser/03-methods/parser.go

@@ -0,0 +1,125 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// domain represents a domain log record
+type domain struct {
+	name   string
+	visits int
+}
+
+// parser parses a log file and provides an iterator to iterate upon the domains
+//
+// the parser struct is carefully crafted to be usable using its zero values except the map field
+type parser struct {
+	sum     map[string]int // visits per unique domain
+	domains []domain       // unique domain names
+	total   int            // total visits to all domains
+	lines   int            // number of parsed lines (for the error messages)
+	err     error          // saves the last error occurred
+}
+
+// newParser creates and returns a new parser.
+//
+// bare func, it doesn't need to operate on a parser value.
+func newParser() *parser {
+	return &parser{sum: make(map[string]int)}
+}
+
+// PUBLIC METHODS (API)
+
+// add parses the given line and saves the result to the internal list of
+// domains. it doesn't add the record when the parsing fails.
+//
+// WRITE METHOD
+func (p *parser) add(line string) {
+	// if there was a previous error do not add
+	if p.err != nil {
+		return
+	}
+
+	dom, err := p.parse(line)
+
+	// store only the last error
+	if err != nil {
+		p.err = err
+		return
+	}
+
+	p.push(dom)
+}
+
+// iterator returns two functions for iterating over domains.
+// next = returns true when there are more domains to iterate on.
+// cur  = returns the current domain
+//
+// READ METHOD
+func (p *parser) iterator() (next func() bool, cur func() domain) {
+	// remember the last received line
+	var last int
+
+	next = func() bool {
+		defer func() { last++ }()
+		return len(p.domains) > last
+	}
+
+	cur = func() domain {
+		d := p.domains[last-1]
+		vis := p.sum[d.name]
+
+		// return a copy so the caller cannot change it
+		return domain{name: d.name, visits: vis}
+	}
+
+	return
+}
+
+// error returns the last error occurred
+//
+// READ METHOD
+func (p *parser) error() error {
+	return p.err
+}
+
+// PRIVATE METHODS
+
+// parse parses the given text and returns a domain struct
+//
+// WRITE METHOD
+func (p *parser) parse(line string) (dom domain, err error) {
+	p.lines++ // increase the parsed line counter (only write is here)
+
+	fields := strings.Fields(line)
+	if len(fields) != 2 {
+		err = fmt.Errorf("wrong input: %v (line #%d)", fields, p.lines)
+		return dom, err
+	}
+
+	name, visits := fields[0], fields[1]
+
+	n, err := strconv.Atoi(visits)
+	if n < 0 || err != nil {
+		err = fmt.Errorf("wrong input: %q (line #%d)", visits, p.lines)
+		return dom, err
+	}
+
+	return domain{name: name, visits: n}, nil
+}
+
+// push pushes the given domain to the internal list of domains.
+// it also increases the total visits for all the domains.
+//
+// WRITE METHOD
+func (p *parser) push(d domain) {
+	// collect the unique domains
+	if _, ok := p.sum[d.name]; !ok {
+		p.domains = append(p.domains, d)
+	}
+
+	p.sum[d.name] += d.visits
+	p.total += d.visits
+}