Text representation of a float formatted using given options.

This BIF solves a problem of float_to_list/1 that doesn't allow
specifying the number of digits after the decimal point when
formatting floats.

    float_to_list(Float, Options) -> string()

    Float = float()
    Options = [Option]
    Option = {decimals, Decimals::0..249} |
             {scientific, Decimals::0..249} |
             compact

Returns a string which corresponds to the text representation of
a `Float` formatted using given options.

When decimals option is specified the returned value will contain
at most `Decimals` number of digits past the decimal point.
When `compact` option is provided the trailing zeros at the end
of the list are truncated (this option is only meaningful together
with the `decimals` option). When `scientific` option is provided,
the float will be formatted using scientific notation with
`Decimals` digits of precision. If `Options` is `[]` the function
behaves like `float_to_list/1`. When using `decimals` option and
the number doesn't fit in the static internal buffer of 256 bytes
the function throws `badarg`.

4 files changed, 74 insertions, 1 deletions

diff --git a/erts/emulator/beam/atom.names b/erts/emulator/beam/atom.names
index c47a608215..b74e2785e5 100644
--- a/erts/emulator/beam/atom.names
+++ b/erts/emulator/beam/atom.names
@@ -144,6 +144,7 @@ atom close
 atom closed
 atom code
 atom command
+atom compact
 atom compat_rel
 atom compile
 atom compressed
@@ -165,6 +166,7 @@ atom current_location
 atom current_stacktrace
 atom data
 atom debug_flags
+atom decimals
 atom delay_trap
 atom dexit
 atom depth
@@ -480,6 +482,7 @@ atom scheduler
 atom scheduler_id
 atom schedulers_online
 atom scheme
+atom scientific
 atom scope
 atom sensitive
 atom sequential_tracer
diff --git a/erts/emulator/beam/bif.c b/erts/emulator/beam/bif.c
index 97c8114437..182129cd36 100644
--- a/erts/emulator/beam/bif.c
+++ b/erts/emulator/beam/bif.c
@@ -2917,7 +2917,73 @@ BIF_RETTYPE float_to_list_1(BIF_ALIST_1)
      need = i*2;
      hp = HAlloc(BIF_P, need);
      BIF_RET(buf_to_intlist(&hp, fbuf, i, NIL));
- }
+}
+
+BIF_RETTYPE float_to_list_2(BIF_ALIST_2)
+{
+    const static int arity_two = make_arityval(2);
+    int decimals = SYS_DEFAULT_FLOAT_DECIMALS;
+    int compact = 0;
+    enum fmt_type_ {
+        FMT_LEGACY,
+        FMT_FIXED,
+        FMT_SCIENTIFIC
+    } fmt_type = FMT_LEGACY;
+    Eterm list = BIF_ARG_2;
+    Eterm arg;
+    int i;
+    Uint need;
+    Eterm* hp;
+    FloatDef f;
+    char fbuf[256];
+
+    /* check the arguments */
+    if (is_not_float(BIF_ARG_1))
+        goto badarg;
+
+    for(; is_list(list); list = CDR(list_val(list))) {
+        arg = CAR(list_val(list));
+        if (arg == am_compact) {
+            compact = 1;
+            continue;
+        } else if (is_tuple(arg)) {
+            Eterm* tp = tuple_val(arg);
+            if (*tp == arity_two && is_small(tp[2])) {
+                decimals = signed_val(tp[2]);
+                if (decimals > 0 && decimals < sizeof(fbuf) - 6 /* "X." ++ "e+YY" */)
+                    switch (tp[1]) {
+                        case am_decimals:
+                            fmt_type = FMT_FIXED;
+                            continue;
+                        case am_scientific:
+                            fmt_type = FMT_SCIENTIFIC;
+                            continue;
+                    }
+            }
+        }
+        goto badarg;
+    }
+    if (is_not_nil(list)) {
+        goto badarg;
+    }
+
+    GET_DOUBLE(BIF_ARG_1, f);
+
+    if (fmt_type == FMT_FIXED) {
+        if ((i = sys_double_to_chars_fast(f.fd, fbuf, sizeof(fbuf),
+                decimals, compact)) <= 0)
+            goto badarg;
+    } else {
+        if ((i = sys_double_to_chars_ext(f.fd, fbuf, sizeof(fbuf), decimals)) <= 0)
+            goto badarg;
+    }
+
+    need = i*2;
+    hp = HAlloc(BIF_P, need);
+    BIF_RET(buf_to_intlist(&hp, fbuf, i, NIL));
+badarg:
+    BIF_ERROR(BIF_P, BADARG);
+}
 
 /**********************************************************************/
 
diff --git a/erts/emulator/beam/bif.tab b/erts/emulator/beam/bif.tab
index 59a91cd40c..a79feb6da3 100644
--- a/erts/emulator/beam/bif.tab
+++ b/erts/emulator/beam/bif.tab
@@ -64,6 +64,7 @@ bif erlang:external_size/1
 bif erlang:external_size/2
 ubif erlang:float/1
 bif erlang:float_to_list/1
+bif erlang:float_to_list/2
 bif erlang:fun_info/2
 bif erlang:garbage_collect/0
 bif erlang:garbage_collect/1
diff --git a/erts/emulator/beam/sys.h b/erts/emulator/beam/sys.h
index 898a30b010..cecaff54a4 100644
--- a/erts/emulator/beam/sys.h
+++ b/erts/emulator/beam/sys.h
@@ -729,9 +729,12 @@ char * getenv_string(GETENV_STATE *);
 void fini_getenv_state(GETENV_STATE *);
 
 /* xxxP */
+#define SYS_DEFAULT_FLOAT_DECIMALS 20
 void init_sys_float(void);
 int sys_chars_to_double(char*, double*);
 int sys_double_to_chars(double, char*, size_t);
+int sys_double_to_chars_ext(double, char*, size_t, size_t);
+int sys_double_to_chars_fast(double, char*, int, int, int);
 void sys_get_pid(char *, size_t);
 
 /* erts_sys_putenv() returns, 0 on success and a value != 0 on failure. */