Theory Log_Nat

(*  Title:      HOL/Library/Log_Nat.thy
    Author:     Johannes Hölzl, Fabian Immler
    Copyright   2012  TU München
*)

section ‹Logarithm of Natural Numbers›

theory Log_Nat
imports Complex_Main
begin

subsection ‹Preliminaries›

lemma divide_nat_diff_div_nat_less_one:
  "real x / real b - real (x div b) < 1" for x b :: nat
proof (cases "b = 0")
  case True
  then show ?thesis
    by simp
next
  case False
  then have "real (x div b) + real (x mod b) / real b - real (x div b) < 1"
    by (simp add: field_simps)
  then show ?thesis
    by (simp add: real_of_nat_div_aux [symmetric])
qed


subsection ‹Floorlog›

definition floorlog :: "nat ⇒ nat ⇒ nat"
  where "floorlog b a = (if a > 0 ∧ b > 1 then nat ⌊log b a⌋ + 1 else 0)"

lemma floorlog_mono: "x ≤ y ⟹ floorlog b x ≤ floorlog b y"
  by (auto simp: floorlog_def floor_mono nat_mono)

lemma floorlog_bounds:
  "b ^ (floorlog b x - 1) ≤ x ∧ x < b ^ (floorlog b x)" if "x > 0" "b > 1"
proof
  show "b ^ (floorlog b x - 1) ≤ x"
  proof -
    have "b ^ nat ⌊log b x⌋ = b powr ⌊log b x⌋"
      using powr_realpow[symmetric, of b "nat ⌊log b x⌋"] ‹x > 0› ‹b > 1›
      by simp
    also have "… ≤ b powr log b x" using ‹b > 1› by simp
    also have "… = real_of_int x" using ‹0 < x› ‹b > 1› by simp
    finally have "b ^ nat ⌊log b x⌋ ≤ real_of_int x" by simp
    then show ?thesis
      using ‹0 < x› ‹b > 1› of_nat_le_iff
      by (fastforce simp add: floorlog_def)
  qed
  show "x < b ^ (floorlog b x)"
  proof -
    have "x ≤ b powr (log b x)" using ‹x > 0› ‹b > 1› by simp
    also have "… < b powr (⌊log b x⌋ + 1)"
      using that by (intro powr_less_mono) auto
    also have "… = b ^ nat (⌊log b (real_of_int x)⌋ + 1)"
      using that by (simp flip: powr_realpow)
    finally
    have "x < b ^ nat (⌊log b (int x)⌋ + 1)"
      by (rule of_nat_less_imp_less)
    then show ?thesis
      using ‹x > 0› ‹b > 1› by (simp add: floorlog_def nat_add_distrib)
  qed
qed

lemma floorlog_power [simp]:
  "floorlog b (a * b ^ c) = floorlog b a + c" if "a > 0" "b > 1"
proof -
  have "⌊log b a + real c⌋ = ⌊log b a⌋ + c" by arith
  then show ?thesis using that
    by (auto simp: floorlog_def log_mult powr_realpow[symmetric] nat_add_distrib)
qed

lemma floor_log_add_eqI:
  "⌊log b (a + r)⌋ = ⌊log b a⌋" if "b > 1" "a ≥ 1" "0 ≤ r" "r < 1"
    for a b :: nat and r :: real
proof (rule floor_eq2)
  have "log b a ≤ log b (a + r)" using that by force
  then show "⌊log b a⌋ ≤ log b (a + r)" by arith
next
  define l::int where "l = int b ^ (nat ⌊log b a⌋ + 1)"
  have l_def_real: "l = b powr (⌊log b a⌋ + 1)"
    using that by (simp add: l_def powr_add powr_real_of_int)
  have "a < l"
  proof -
    have "a = b powr (log b a)" using that by simp
    also have "… < b powr floor ((log b a) + 1)"
      using that(1) by auto
    also have "… = l"
      using that by (simp add: l_def powr_real_of_int powr_add)
    finally show ?thesis by simp
  qed
  then have "a + r < l" using that by simp
  then have "log b (a + r) < log b l" using that by simp
  also have "… = real_of_int ⌊log b a⌋ + 1"
    using that by (simp add: l_def_real)
  finally show "log b (a + r) < real_of_int ⌊log b a⌋ + 1" .
qed

lemma floor_log_div:
  "⌊log b x⌋ = ⌊log b (x div b)⌋ + 1" if "b > 1" "x > 0" "x div b > 0"
    for b x :: nat
proof-
  have "⌊log b x⌋ = ⌊log b (x / b * b)⌋" using that by simp
  also have "… = ⌊log b (x / b) + log b b⌋"
    using that by (subst log_mult) auto
  also have "… = ⌊log b (x / b)⌋ + 1" using that by simp
  also have "⌊log b (x / b)⌋ = ⌊log b (x div b + (x / b - x div b))⌋" by simp
  also have "… = ⌊log b (x div b)⌋"
    using that real_of_nat_div4 divide_nat_diff_div_nat_less_one
    by (intro floor_log_add_eqI) auto
  finally show ?thesis .
qed

lemma compute_floorlog [code]:
  "floorlog b x = (if x > 0 ∧ b > 1 then floorlog b (x div b) + 1 else 0)"
  by (auto simp: floorlog_def floor_log_div[of b x] div_eq_0_iff nat_add_distrib
    intro!: floor_eq2)

lemma floor_log_eq_if:
  "⌊log b x⌋ = ⌊log b y⌋" if "x div b = y div b" "b > 1" "x > 0" "x div b ≥ 1"
    for b x y :: nat
proof -
  have "y > 0" using that by (auto intro: ccontr)
  thus ?thesis using that by (simp add: floor_log_div)
qed

lemma floorlog_eq_if:
  "floorlog b x = floorlog b y" if "x div b = y div b" "b > 1" "x > 0" "x div b ≥ 1"
    for b x y :: nat
proof -
  have "y > 0" using that by (auto intro: ccontr)
  then show ?thesis using that
    by (auto simp add: floorlog_def eq_nat_nat_iff intro: floor_log_eq_if)
qed

lemma floorlog_leD:
  "floorlog b x ≤ w ⟹ b > 1 ⟹ x < b ^ w"
  by (metis floorlog_bounds leD linorder_neqE_nat order.strict_trans power_strict_increasing_iff
      zero_less_one zero_less_power)

lemma floorlog_leI:
  "x < b ^ w ⟹ 0 ≤ w ⟹ b > 1 ⟹ floorlog b x ≤ w"
  by (drule less_imp_of_nat_less[where 'a=real])
    (auto simp: floorlog_def Suc_le_eq nat_less_iff floor_less_iff log_of_power_less)

lemma floorlog_eq_zero_iff:
  "floorlog b x = 0 ⟷ b ≤ 1 ∨ x ≤ 0"
  by (auto simp: floorlog_def)

lemma floorlog_le_iff:
  "floorlog b x ≤ w ⟷ b ≤ 1 ∨ b > 1 ∧ 0 ≤ w ∧ x < b ^ w"
  using floorlog_leD[of b x w] floorlog_leI[of x b w]
  by (auto simp: floorlog_eq_zero_iff[THEN iffD2])

lemma floorlog_ge_SucI:
  "Suc w ≤ floorlog b x" if "b ^ w ≤ x" "b > 1"
  using that le_log_of_power[of b w x] power_not_zero
  by (force simp: floorlog_def Suc_le_eq powr_realpow not_less Suc_nat_eq_nat_zadd1
      zless_nat_eq_int_zless int_add_floor less_floor_iff
      simp del: floor_add2)

lemma floorlog_geI:
  "w ≤ floorlog b x" if "b ^ (w - 1) ≤ x" "b > 1"
  using floorlog_ge_SucI[of b "w - 1" x] that
  by auto

lemma floorlog_geD:
  "b ^ (w - 1) ≤ x" if "w ≤ floorlog b x" "w > 0"
proof -
  have "b > 1" "0 < x"
    using that by (auto simp: floorlog_def split: if_splits)
  have "b ^ (w - 1) ≤ x" if "b ^ w ≤ x"
  proof -
    have "b ^ (w - 1) ≤ b ^ w"
      using ‹b > 1›
      by (auto intro!: power_increasing)
    also note that
    finally show ?thesis .
  qed
  moreover have "b ^ nat ⌊log (real b) (real x)⌋ ≤ x" (is "?l ≤ _")
  proof -
    have "0 ≤ log (real b) (real x)"
      using ‹b > 1› ‹0 < x›
      by auto
    then have "?l ≤ b powr log (real b) (real x)"
      using ‹b > 1›
      by (auto simp flip: powr_realpow intro!: powr_mono of_nat_floor)
    also have "… = x" using ‹b > 1› ‹0 < x›
      by auto
    finally show ?thesis
      unfolding of_nat_le_iff .
  qed
  ultimately show ?thesis
    using that
    by (auto simp: floorlog_def le_nat_iff le_floor_iff le_log_iff powr_realpow
        split: if_splits elim!: le_SucE)
qed


subsection ‹Bitlen›

definition bitlen :: "int ⇒ int"
  where "bitlen a = floorlog 2 (nat a)"

lemma bitlen_alt_def:
  "bitlen a = (if a > 0 then ⌊log 2 a⌋ + 1 else 0)"
  by (simp add: bitlen_def floorlog_def)

lemma bitlen_zero [simp]:
  "bitlen 0 = 0"
  by (auto simp: bitlen_def floorlog_def)

lemma bitlen_nonneg:
  "0 ≤ bitlen x"
  by (simp add: bitlen_def)

lemma bitlen_bounds:
  "2 ^ nat (bitlen x - 1) ≤ x ∧ x < 2 ^ nat (bitlen x)" if "x > 0"
proof -
  from that have "bitlen x ≥ 1" by (auto simp: bitlen_alt_def)
  with that floorlog_bounds[of "nat x" 2] show ?thesis
    by (auto simp add: bitlen_def le_nat_iff nat_less_iff nat_diff_distrib)
qed

lemma bitlen_pow2 [simp]:
  "bitlen (b * 2 ^ c) = bitlen b + c" if "b > 0"
  using that by (simp add: bitlen_def nat_mult_distrib nat_power_eq)

lemma compute_bitlen [code]:
  "bitlen x = (if x > 0 then bitlen (x div 2) + 1 else 0)"
  by (simp add: bitlen_def nat_div_distrib compute_floorlog)

lemma bitlen_eq_zero_iff:
  "bitlen x = 0 ⟷ x ≤ 0"
  by (auto simp add: bitlen_alt_def)
   (metis compute_bitlen add.commute bitlen_alt_def bitlen_nonneg less_add_same_cancel2
      not_less zero_less_one)

lemma bitlen_div:
  "1 ≤ real_of_int m / 2^nat (bitlen m - 1)"
    and "real_of_int m / 2^nat (bitlen m - 1) < 2" if "0 < m"
proof -
  let ?B = "2^nat (bitlen m - 1)"

  have "?B ≤ m" using bitlen_bounds[OF ‹0 <m›] ..
  then have "1 * ?B ≤ real_of_int m"
    unfolding of_int_le_iff[symmetric] by auto
  then show "1 ≤ real_of_int m / ?B" by auto

  from that have "0 ≤ bitlen m - 1" by (auto simp: bitlen_alt_def)

  have "m < 2^nat(bitlen m)" using bitlen_bounds[OF that] ..
  also from that have "… = 2^nat(bitlen m - 1 + 1)"
    by (auto simp: bitlen_def)
  also have "… = ?B * 2"
    unfolding nat_add_distrib[OF ‹0 ≤ bitlen m - 1› zero_le_one] by auto
  finally have "real_of_int m < 2 * ?B"
    by (metis (full_types) mult.commute power.simps(2) of_int_less_numeral_power_cancel_iff)
  then have "real_of_int m / ?B < 2 * ?B / ?B"
    by (rule divide_strict_right_mono) auto
  then show "real_of_int m / ?B < 2" by auto
qed

lemma bitlen_le_iff_floorlog:
  "bitlen x ≤ w ⟷ w ≥ 0 ∧ floorlog 2 (nat x) ≤ nat w"
  by (auto simp: bitlen_def)

lemma bitlen_le_iff_power:
  "bitlen x ≤ w ⟷ w ≥ 0 ∧ x < 2 ^ nat w"
  by (auto simp: bitlen_le_iff_floorlog floorlog_le_iff)

lemma less_power_nat_iff_bitlen:
  "x < 2 ^ w ⟷ bitlen (int x) ≤ w"
  using bitlen_le_iff_power[of x w]
  by auto

lemma bitlen_ge_iff_power:
  "w ≤ bitlen x ⟷ w ≤ 0 ∨ 2 ^ (nat w - 1) ≤ x"
  unfolding bitlen_def
  by (auto simp flip: nat_le_iff intro: floorlog_geI dest: floorlog_geD)

lemma bitlen_twopow_add_eq:
  "bitlen (2 ^ w + b) = w + 1" if "0 ≤ b" "b < 2 ^ w"
  by (auto simp: that nat_add_distrib bitlen_le_iff_power bitlen_ge_iff_power intro!: antisym)

end