From 9e07c6adc7a0616eec40e78024a8501ec7d96559 Mon Sep 17 00:00:00 2001
From: Bruce Hill <bruce@bruce-hill.com>
Date: Sun, 18 Aug 2024 23:31:36 -0400
Subject: [PATCH] Add Channel:peek()

---
 api/channels.md    | 26 ++++++++++++++++++++++++++
 builtins/channel.c | 10 ++++++++++
 builtins/channel.h |  2 ++
 compile.c          |  4 ++++
 test/threads.tm    |  5 ++++-
 typecheck.c        |  1 +
 6 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/api/channels.md b/api/channels.md
index dbb6707..ca7ce17 100644
--- a/api/channels.md
+++ b/api/channels.md
@@ -91,6 +91,32 @@ get(channel:|T|) -> T
 **Returns:**  
 The item removed from the channel.
 
+**Example:**  
+```markdown
+>> channel:peek()
+= "Hello"
+```
+
+---
+
+### `peek`
+
+**Description:**  
+Returns the next item that will come out of the channel, but without removing
+it. If the channel is empty, it waits until an item is available.
+
+**Usage:**  
+```markdown
+peek(channel:|T|) -> T
+```
+
+**Parameters:**
+
+- `channel`: The channel from which to remove an item.
+
+**Returns:**  
+The item removed from the channel.
+
 **Example:**  
 ```markdown
 >> channel:get()
diff --git a/builtins/channel.c b/builtins/channel.c
index f47a0b1..9360bcf 100644
--- a/builtins/channel.c
+++ b/builtins/channel.c
@@ -68,6 +68,16 @@ public void Channel$get(channel_t *channel, void *out, int64_t item_size, int64_
     (void)pthread_cond_signal(&channel->cond);
 }
 
+public void Channel$peek(channel_t *channel, void *out, int64_t item_size)
+{
+    (void)pthread_mutex_lock(&channel->mutex);
+    while (channel->items.length == 0)
+        pthread_cond_wait(&channel->cond, &channel->mutex);
+    memcpy(out, channel->items.data, item_size);
+    (void)pthread_mutex_unlock(&channel->mutex);
+    (void)pthread_cond_signal(&channel->cond);
+}
+
 public array_t Channel$view(channel_t *channel)
 {
     (void)pthread_mutex_lock(&channel->mutex);
diff --git a/builtins/channel.h b/builtins/channel.h
index 3b2d2c1..b828237 100644
--- a/builtins/channel.h
+++ b/builtins/channel.h
@@ -15,6 +15,8 @@ void Channel$give(channel_t *channel, const void *item, int64_t padded_item_size
 void Channel$give_all(channel_t *channel, array_t to_give, int64_t padded_item_size);
 void Channel$get(channel_t *channel, void *out, int64_t item_size, int64_t padded_item_size);
 #define Channel$get_value(channel, t, padded_item_size) ({ t _val; Channel$get(channel, &_val, sizeof(t), padded_item_size); _val; })
+void Channel$peek(channel_t *channel, void *out, int64_t item_size);
+#define Channel$peek_value(channel, t) ({ t _val; Channel$peek(channel, &_val, sizeof(t)); _val; })
 void Channel$clear(channel_t *channel);
 array_t Channel$view(channel_t *channel);
 uint32_t Channel$hash(const channel_t **channel, const TypeInfo *type);
diff --git a/compile.c b/compile.c
index 08b5693..477913c 100644
--- a/compile.c
+++ b/compile.c
@@ -2377,6 +2377,10 @@ CORD compile(env_t *env, ast_t *ast)
                 CORD self = compile_to_pointer_depth(env, call->self, 0, false);
                 (void)compile_arguments(env, ast, NULL, call->args);
                 return CORD_all("Channel$get_value(", self, ", ", compile_type(item_t), ", ", padded_item_size, ")");
+            } else if (streq(call->name, "peek")) {
+                CORD self = compile_to_pointer_depth(env, call->self, 0, false);
+                (void)compile_arguments(env, ast, NULL, call->args);
+                return CORD_all("Channel$peek_value(", self, ", ", compile_type(item_t), ")");
             } else if (streq(call->name, "clear")) {
                 CORD self = compile_to_pointer_depth(env, call->self, 0, false);
                 (void)compile_arguments(env, ast, NULL, call->args);
diff --git a/test/threads.tm b/test/threads.tm
index e8d83f4..f7acd01 100644
--- a/test/threads.tm
+++ b/test/threads.tm
@@ -2,6 +2,10 @@ enum Job(Increment(x:Int), Decrement(x:Int))
 
 func main():
     jobs := |:Job; max_size=2|
+    >> jobs:give(Increment(5))
+    >> jobs:peek()
+    = Job.Increment(x=5)
+
     results := |:Int; max_size|
     >> thread := Thread.new(func():
         //! In another thread!
@@ -13,7 +17,6 @@ func main():
                 >> results:give(x-1)
     )
 
-    >> jobs:give(Increment(5))
     >> jobs:give(Decrement(100))
     >> jobs:give(Decrement(100))
     >> jobs:give(Decrement(100))
diff --git a/typecheck.c b/typecheck.c
index 805f83e..2fb8556 100644
--- a/typecheck.c
+++ b/typecheck.c
@@ -774,6 +774,7 @@ type_t *get_type(env_t *env, ast_t *ast)
             else if (streq(call->name, "get")) return Match(self_value_t, ChannelType)->item_type;
             else if (streq(call->name, "give")) return Type(VoidType);
             else if (streq(call->name, "give_all")) return Type(VoidType);
+            else if (streq(call->name, "peek")) return Match(self_value_t, ChannelType)->item_type;
             else if (streq(call->name, "view")) return Type(ArrayType, .item_type=Match(self_value_t, ChannelType)->item_type);
             else code_err(ast, "There is no '%s' method for arrays", call->name);
         }