Add command history and cycling

Aerc will keep track of the previous 1000 commands, which the user can
cycle through using the arrow keys while in the ex-line. Pressing up
will move backwards in history while pressing down will move forward.

1 files changed, 62 insertions, 0 deletions

diff --git a/commands/history.go b/commands/history.go
new file mode 100644
index 00000000..77bb1553
--- /dev/null
+++ b/commands/history.go
@@ -0,0 +1,62 @@
+package commands
+
+type cmdHistory struct {
+	// rolling buffer of prior commands
+	//
+	// most recent command is at the end of the list,
+	// least recent is index 0
+	cmdList []string
+
+	// current placement in list
+	current int
+}
+
+// number of commands to keep in history
+const cmdLimit = 1000
+
+// CmdHistory is the history of executed commands
+var CmdHistory = cmdHistory{}
+
+func (h *cmdHistory) Add(cmd string) {
+	// if we're at cap, cut off the first element
+	if len(h.cmdList) >= cmdLimit {
+		h.cmdList = h.cmdList[1:]
+	}
+
+	h.cmdList = append(h.cmdList, cmd)
+
+	// whenever we add a new command, reset the current
+	// pointer to the "beginning" of the list
+	h.Reset()
+}
+
+// Prev returns the previous command in history.
+// Since the list is reverse-order, this will return elements
+// increasingly towards index 0.
+func (h *cmdHistory) Prev() string {
+	if h.current <= 0 || len(h.cmdList) == 0 {
+		h.current = -1
+		return "(Already at beginning)"
+	}
+	h.current--
+
+	return h.cmdList[h.current]
+}
+
+// Next returns the next command in history.
+// Since the list is reverse-order, this will return elements
+// increasingly towards index len(cmdList).
+func (h *cmdHistory) Next() string {
+	if h.current >= len(h.cmdList)-1 || len(h.cmdList) == 0 {
+		h.current = len(h.cmdList)
+		return "(Already at end)"
+	}
+	h.current++
+
+	return h.cmdList[h.current]
+}
+
+// Reset the current pointer to the beginning of history.
+func (h *cmdHistory) Reset() {
+	h.current = len(h.cmdList)
+}